In pokemon yellow, you take to steps on the grass, and Pro. Oak comes up to stop you, as soon as he walks on the grass, he gets into a battle with a pikachu and catches it. Then he will take you back to the lab, when you get there, there will be one pokeball on the table, and Gary will be in front of it, Gary will get that pokeball(with an eevee inside) and Pro. Oak will give you the pikachu he caught. Gary changes his pokemon thru the game, rather than keeping the same ones, and adding new ones to those. I'm not sure what happens after you beat the elite four(I haven't gotten that far yet. Officer Jenny is the one that will give you the three main pokemon(she is in three different towns), you cannot catch them. In the Viridian City Poke Center, there is a jigglypuff, if you face him and press A, he will sing, pikachu will fall asleep(since he isn't in a pokeball).
I'll update this when I find out more.
