What pokemon are most effective?
^^^ Thats a link to all the top tier pokemon. Any of those are likely to be safe bets.
What number of walls, sweepers etc should I shoot for?
The widely accepted team composition is as follows:
1 Physical Sweeper
1 Special Sweeper
1 Physical Wall
1 Special Wall
2 Support Slots
The first 4 are the backbone of any balanced team, while the support slots are more flexible. Types of pokes that fall into that category are:
Let me know if you want definitions.
How many of each type to use
There is no set rule when it comes to this kind of thing, although monotype teams always fail lol. The key here is balance. Your pokes should be aiming to cover each others weaknesses. So if you have two fire types, ensure that you have at least one water, ground and rock resistance to back them up- although not necersarrily in the same poke ;) after a few battles any glaring weaknesses quickly become apparant if your team is imbalanced. Download the program from www.shoddybattle.com
, it's a battle simulator so you can test teams before you build them on wifi.
Let me know if you have any other questions =]