Okay, I'll try my best.
You need a good coverage in your team. Best thing is to have no more than two pokemons who are weak to the same type. For a good coverage teach your pokemon different types of moves. For example, ice beam on a water pokemon is very good, the possibilities are endless. When you have a team, it is good to make a list of every 17(18) types and look if you have something against each one in your team.
Look for pokemon with good attacks, stats and also abilities on sites like bulbapedia.
Possibly one of the easiest way to set up your team is to give your Pokemon 2 attacks (different types if possible), one move to increase your stats (Calm Mind, Swords Dance, Dragon Dance ...) and another attack like Toxic, Will-o-wisp or Thunderwave, but you can also choose Protect or something like that.
An example: Guardevoir; Psychic, Shadow Ball, Thunder Wave, Calm Mind .. thats a simple one. Psychic as a very good STAB-Move, Shadow Ball as a coverage move(you can put in what ever you want(i.e. Thunderbolt or Energy Ball)), Thunder Wave to paralyse the foe and Calm Mind to boost yourself.
But you have to try different pokemon, different team combinations and attacks, figure it out for yourself which pokemon are the best ones for you
Hope I could help