Ok I admit, scouting isn't as easy as it sounds. Most of the time you have your mind occupied with more important things. However scouting does reveal a lot of your opponents plans, and it's not something that he/she wants you to do.
Since Terrans do have the option to build an academy or a factory, it helps to decide which will be more effective and win the game faster. Scouting also helps to defend against tech jumps, as someone trying to reaver drop, or dark templar rush. These attackes will never work once you see that early cybernetics core, or citadel. Also scouting helps to break up those offensive bunkers that your opponent was hoping to kill you with.
Now since you don't start off with any air units, which by the way are good scouts, you'll have to improvise. Using an scv to run around the map is the best idea early in the game, for there cost, production speed, and they do move faster and have more HP than a marine.
One trick is to run into someones base early in the game, and then fake that you're going to leave, except that you hang around a bit more somewhere in a corner where you won't be seen. You can then use the same scv to run back into the base a minute later to see what's going on. SCVs do have more HP making it a good idea to attack the workers, forcing them off the mineral patches and onto your SCV. Also having a marine standing guard outisde of your enemy's base is a good idea, since you can just run back home once he decides to attack.
You must scout, in order to know where to attack. You shouldn't gather up a army then scout, because it's more likely that your opponent already has units as well, capable of killing that scout before it revealed anything. Especially on a 1 on 1 game, knowing everything is a good idea. If you see a robotics then you know a shuttle is coming soon, if you see a factory, then tanks become a threat, if a spire is present then you must look for that expansion that he has, etc.
Well once the comstat becomes available, you shouldn't start saving up on energy. Try to use it right away on your opponents base to see what units he has, and what he plans to attack you with. Comstat is also a great way for scouting islands, and is also a necessity for marching up and bunkering someone in. Also most building that the Terrans have can lift off, which are great for scouting, or at least extending sight to siege tanks.
Vultures are also great scouts. They are the fastest unit in the game, making them great units to run around, constantly scouting any empty expansion sites. They also have spider mines which are good to place around key areas where you want to see. Spots as in front of the opponents base and at expansions are a good idea. It helps to prevent dark templars from running out and doing damage onto your base. On maps as hunters, they're good to run around to make sure any of the 6 expansions left aren't taken.
Wraiths are also great units to scout with. Usually I take a good wraith with plenty of energy, to cloak and scout around. They're important to use since Terran's don't have too many options for scouting. Try using wraiths as observers, by cloaking a few and using them to reveal all the fog in front of your base, but prefferably off the main pathway, since they will fire at any targets. Ghosts to some extent make good scouts as well, but it's so late in the game, that you should have 2-4 comstats ready to scan at any expansion that you feel your opponent will take.
Remember to always scan..... and never save up to 200 energy.
