"So what do I do to make a team," you ask. The first thing you do is you pick your Pokemon.
A good team of Pokemon is balanced to make up for any weakness. No matter what they do in the game, going out with six Gyarados is NOT a good idea, especially if your opponent has at least one electric Pokemon!
Which Pokemon you decide to use also depends on what you want them to do. Here are some of the ways that Pokemon can be trained:
Pokemon Roles
Here are the roles a Pokemon can play, note there are many out there, this guide covers the most basic and essential.
Sweeper- A Pokemon who's mostly meant to attack. Depending on which stat is better, you can either have a Physical Sweeper or a Special Sweeper. Sometimes, the Physical and Special Stats are close enough that you can make the Pokemon a Mixed Sweeper, with both Physical AND Special attacks. Sweepers in general also have a lot of speed on them, so they can get their hits in before the other guy does.
Hazer- Haze is a move that makes ALL Stats go back to normal. For instance, if your opponent has used Swords Dance to increase their Attack a lot, Haze will make sure that all that Sword-dancing was for nothing. Hazers tend to have high Defense, in case the opponent has already used Swords dance a couple of times. Any sort of "Shuffler" move, like Roar or Whirlwind, can also be used, because usually when a Pokemon switches out, their Stats go back to normal.
Tank- Usually a Pokemon with good Defense, Tanks are MADE to take hits. Tanks have recovering moves, like Rest, and items that recover hit points. There are a couple of variations of this. The wall is meant to take any kind of Physical hit, while the sponge is meant to take all kinds of Special Attacks.
Choice Bander/CB-er- A Pokemon holding the item Choice Band. Choice Band increases the power of your moves, but you can only use one move...until you switch out, at least!
Stallers- Stallers are like the tanks, but they have moves that are meant to slowly kill the opponent, like Toxic and Will-o-Wisp.
Spiker- A spiker is a Pokemon who has either the move Spikes or Toxic Spikes.
Healer/Cleric- These Pokemon are for healing others. They have moves like Aromatherapy and Wish.
SunnyBeamer- A Pokemon that uses Sunny Day to get some one-shot Solar Beams.
Shuffler- A Shuffler (Also known as a Toxi/Pyro/Parashuffler, depending on what you use) is a Pokemon with some kind of move to make your opponent burned, poisoned, or paralyzed. They also have a move that forces the opponent to switch out, like Roar or Whirlwind. When that is done, they move on to the next Pokemon the opponent brings out If it's done correctly, the opponent's entire team could end up burned, poisoned, or paralyzed.
Trappers- Trappers are Pokemon with either the moves Mean Look or Fire Spin, or the abilities Shadow Tag or Arena Trap. They're made to keep the opponent from switching out. They usually have either a Poisoning or a Burning move.
SkarmBliss- Two of the most overused defensive Pokemon in the game. Skarmory has some VERY good resistances, as well as high physical Defense. Blissey is the ultimate Special Defense Pokemon, with horribly high Hit Points to boot. Between the two of them, they can stop most hits from getting through, which is why they're so overused.
Substitute- Substitute is a move where a Pokemon makes a clone using 1/8th of its' Hit Points. The Clone, or Sub, has the exact same Ability and Stats as the Pokemon that made it, but fewer Hit Points. So a Sub from a Pokemon who has high Defenses will be able to take more hits than a Pokemon who has lower Defenses. When the Sub takes too much damage, it fades away. This is a very good Stalling move, especially when combined with Leftovers. It also means that a Sub can take a hit that would knock out the Pokemon that made it, but the Pokemon would only be missing a bit of Health. And if the Sub survives until the next turn, the Pokemon can take the time to do all sorts of nasty things, like wind up for a Focus Punch, use any sort of Stat Booster, use Stealth Rock, Spikes or Toxic Spikes to make it harder to Switch, and so on. Subs can be Baton Passed to the next Pokemon. They disappear when the Pokemon that made them Switches, or is Roared away.