When you first start the game, you'll be in your room. Exit the room and
of the house. Then, attempt to go up the road to Viridian City. You'll be
stopped by Professor Oak. He'll lead you to his lab and will tell you to pick a Pokémon. That
Pokémon is Eevee. But when you try to pick it, Gary will run forward and shove
you away, claiming Eevee for himself. Professor Oak will see this and will give
you the Pikachu he just caught. Then, Gary will challenge you to a battle.
Beat him to get experience points.
Viridian Forest is full of tall grass and wild Pokémon. You'll have to be very careful when you navigate its many forks and turns. Some trainers are also lurking and will challenge you to battle if they see you. Make your way through by following the road east (right), north (up), west (left), south (down), west (left), north (up), west (left), south (down), west (left), then north (up) all the way to the exit. You'll still need to follow that road north a bit before you can see Pewter City.
In Pewter City,
you will encounter your first gym (ok, maybe your second). The leader is Brock,
a rock/ground type specialist. Make sure that you train your Pokémon to at
least level 14, before you try to battle. He has a level 12 Geodude and a level
14 Onix. When you beat him, he'll turn over a Boulder Badge.
After you get
the ticket, go back to Cerulean City and look for the gym. The gym leader is
Misty. She specializes in water type Pokémon. Her Pokémon include a level 18
Staryu and a level 21 Starmie. If you chose a Charmander as your first Pokémon,
your in big trouble. Otherwise, level up your Pokémon to level 22 and start
battling. If you win, she'll give you a Cascade Badge.
This gym is controlled by Lt. Surge. Lt. Surge will be in a room behind locked doors. However, you can open the doors by checking the trashcans littered on the gym floor. Some of the trashcans contain hidden switches. Find two of them to open the doors. When you find one, though, you'll have to find the second one next. Otherwise, the switches will reset and you'll have to look for them all over again. After you open the doors, you'll see Lt. Surge. He specializes in electric type Pokémon, including a level 21 Voltorb, a level 18 Pikachu, and a level 24 Raichu. When you beat him, he'll hand over his Thunder Badge.
After you get
the Thunder Badge, you should go through Diglett's Cave. Diglett's Cave is full
of Digletts and some occasional Dugtrios. At the end, there will be a house.
Inside, you'll find HM 5, Flash. Flash is used to light dark caves, so you can
Rock Tunnel has 2 floors. It is not very difficult to navigate, though. Just follow the path and after several ladders, you'll come to a final ladder with a sign near it. That ladder leads to the exit.
You'll end up in Lavender Town. Lavender Town doesn't have anything in particular, unless you count the Pokémon Tower. But you don't need to get through the Pokémon Tower yet. You just need to use Lavender Town's Underground Tunnel to go to Celadon City.
In Celadon City, there is a place called Game Corner. There are some suspicious activities going on, so check it out. Look for the guy in a black suit standing in the back of Game Corner room. Talk to him. He'll challenge you to a battle. If you beat him, he'll leave. Check out the poster behind him. It'll review a secret switch. Press it to make a secret set of stairs appear. Go up the stairs. It seems like Team Rocket has taken over the building. Climb all the way to the top to meet Giovanni. He'll be guarded by some other Rockets. Defeat them to get to Giovanni. Giovanni will challenge you to a battle. He will have an Onix , a Rhyhorn, and a Kangaskhan. All of his Pokémon are between levels 25 and 27. Beat him to make him leave. He will leave behind the Silph Scope, used to see ghosts.
When Giovanni has left, get out of the building and go to the gym. Erika is the leader. She specializes in grass type Pokémon. Some of her Pokémon include a level 29 Victreebel, a level 24 Tangela, and a level 29 Vileplume. After beating her, she'll give you a Rainbow Badge.
Now, go back to Lavender Town. Team Rocket has now taken over Pokémon Tower and has also taken Mr. Fuji as a hostage. When you get there, Gary will be waiting for you. He'll challenge you to a battle. He will have a level 25 Pidgeotto, a level 23 Gyarados, a level 22 Growlithe, a level 20 Kadabra and the Pokémon he chose in the beginning. When you beat him, continue to climb up the floors. On the fifth floor, there will be a healing zone. Step on it to heal all of your Pokémon. On the last flight of stairs to the seventh floor, you'll encounter the ghost of a Marowak. Defeat the ghost Marowak to continue to the seventh floor. When you get to the seventh floor, you'll meet Team Rocket. Beat them all to rescue Mr. Fuji. Mr. Fuji will lead you back to his house and will give you a Poké Flute. A Poké Flute will wake up any sleeping Pokémon.
Now, go back to Celadon City and buy a drink (any kind). Give it to the guard that's guarding Saffron City. After you give it to him, he'll let you through. Team Rocket has now taken over Silph Co. Stop them by going to the building's top floor. But first, you'll encounter Gary. Gary'll have a Pidgeot, an Alakazam, and a Growlithe. All of them will be between the levels 35 and 40. Afterwards, he'll leave and you'll be able to go on. On the highest floor, you'll meet Giovanni again. This time, he has taken the president of Silph Co. hostage. Giovanni will have a Nidorino, a Rhyhorn, a Nidoqueen, and a Kangaskhan. Defeat him to free the president. He will give you a Master Ball, the best Pokéball in the game.
After defeating Giovanni and saving Silph Co., you should head out of the building and go to the gym. There are 2 gyms, but the real one is controlled by Sabrina. Sabrina's specialty is psychic type Pokémon. She has a level 38 Kadabra, a level 37 Mr. Mime, a level 38 Venomoth, and a level 43 Alakazam. When you beat her, she'll give you the Marsh Badge.
The other gym
in Saffron City is actually a fighting dojo. If you beat the leader there, he'll
reward you with either a Hitmonlee or a Hitmonchan (you pick). Both of them are
very rare fighting Pokémon.
After you get
HM 4, go to the local gym. The gym leader is Koga, a poison type specialist. He
has a level 37 Koffing, a level 39 Muk, another level 37 Koffing, and a level 43
Weezing. When you win, he'll reward you with a Soul Badge.
When you get to Cinnabar Island, the gym will be locked. Look for the key in the burned Pokémon Mansion. The burned mansion requires you to find hidden switches in statues. These switches switch open an lock certain doors. Use this technique to get the key.
Take the key to the gym door. Inside, you'll encounter a series of rooms blocked by a door. Inside each room is a trainer. You can unlock the door by either beating the trainer or answering a question asked by the machine in the back of the room. If you answer correctly, the door will open. If you don't, the trainer will come and battle you. The leader of the gym is Blaine. He is a fire type specialist. His Pokémon are a level 42 Growlithe, a level 40 Ponyta, a level 42 Rapidash, and a level 47 Arcanine. If you win, he'll hand over a Volcano Badge.
Now get ready for a very tough battle! The 1st of the Elite 4 is Lorelei. Her Pokémon are all ice types. They include a level 54 Dewgong, a level 53 Cloyster, a level 54 Slowbro, a level 56 Jynx, and a level 56 Lapras. You should use a fire, fighting , or rock type to pulverize her team.
The next of the Elite 4 is Bruno. His special is fighting type. His arsenal includes a level 53 Onix, a level 55 Hitmonchan, a level 55 Hitmonlee, a level 56 Onix, and a level 58 Machamp. You can cut them down to size with a flying or psychic type.
Agatha is the next Elite 4 member. Her specialty is ghost type Pokémon. It includes a level 56 Gengar, a level 56 Golbat, a level 55 Haunter, a level 58 Arbok, and a level 60 Gengar. You should use a psychic type, a ground type, or a fire type to beat them.
The last member of the Elite 4 is Lance. He specializes in dragon types. His team has a level 58 Gyarados, a level 56 Dragonair, another level 56 Dragonair, a level 60 Aerodactyl, and a level 62 Dragonite. A high level ice or fighting type will do the trick.
There will be a surprise when you beat all of the Elite 4 members. It seems that Gary has already beaten the Elite 4. You'll have to defeat him too to be the true Pokémon League champion. Gary's arsenal includes a level 61 Pidgeot, a level 59 Alakazam, a level 61 Rhydon, and the other 3 Pokémon will be depending on what you chose for your starting Pokémon. If you chose Charmander, his last 3 Pokémon will be a level 61 Cloyster, a level 63 Ninetales, and a level 65 Blastoise. If you chose Squirtle, his Pokémon will be a level 61 Gyarados, a level 63 Arcanine, and a level 65 Venusaur. If you chose Bulbasaur, his Pokémon will be a level 61 Exeggcute, a level 63 Gyarados, and a level 65 Charizard. After you beat Gary, you'll be declared the official Pokémon League champion. Professor Oak will take you to the champion's room and you'll get to see the credits.
After you see the credits, the game will restart. Continue your game and fly to Cerulean City. You'll see that the person previously blocking the Unknown Dungeon would have disappeared. You can now enter it. Inside, you'll discover the most powerful Pokémon in the game. Deep in the dungeon will be Mewtwo. Mewtwo is the most powerful Pokémon in the game (besides Mew). If you saved your Master Ball, then you should definitely use it for Mewtwo. But if you didn't, then all I can say to you is, good luck! You'll need it!