ok, step one: building a balanced team
By balanced I mean a few things
1. Cover a lot of types (make sure your team isn't all weak to one type incoming and that you're not really ineffective offensively against annother)
2. Have some pokemon with good special attack and some with good normal attack (more on this later) and similarly some with good defense and some with good special defense (and maybe one with high health)
3. Make sure you have a clear strategy with each of your pokemon individually (unless you're using a complex synergy strategy, but that's annother thing entirely)
Step 2, advanced training:
You're going to want to look into something called EV training
EV training is like this:
Every pokemon has a certain EV, or effort value, associated with beating them in battle. These EV's coorespond to a specific stat, like speed, or attack, or special attack etc. Beating a bunch of pokemon who reward you with speed EVs will make your pokemon who beat them faster. Every pokemon you have can only get a certain number of EVs at any time during leveling and thuse you can EV train all your EVs at level 99, or at level 1 (hypothetically).
Now, to determine what type of EVs you want and where to get them depends on your team. First you'll want to look at your pokemon's natures and look them up on bulbapedia. Certain natures do things like "reduce speed, but increase attack." On these pokemon you'll want to find attack EVs (it's better to have a pokemon that excells in one area, than one that is mediocare in all areas).
Then use bulbapedia again to find the list of which pokemon give which EVs and then you go find those pokemon in game and beat them.
Lastly:
Try testing your team out against trainers in game, or friends, or PM me for my wi-fi code and we'll battle. Like I said, I'm a pokemon Professor.
When I train my teams, I usually skip step 2, and still do rather ok. Step one is the important one. Just find a team you like that is rather diverse and you'll be fine.