You will first post a random number generator from 1-5.
Every time you grow a level, you gain 1 base attack and 10 HP, and rarely, special abilities and evolutions. Also, you need to update your level status and hp status and multiplier status, and level EXP, and Total EXP, and your ability status, and evolution statuses, which are all displayed on the Pikachu Records Subforum.
Then, you will battle the pokemon that the number is. For example in a list like this Pikachu would be #5:
Rattatta
Pidgey
Shinx
Bidoof
Pikachu
This is the Power and damage chart:
Power 1-20=Damage X1
Power 21-40=Damage X3
Power 41-60=Damage X6
Power 61-80=Damage X9
Power 81-99=Damage X10
Power 100=DamageX15

Also, you always go first, to eliminate confusion. If you defeat the opponent and you deal more damage than was required to defeat the enemy, then you gain the extra EXP for the negative amount of HP the enemy has. For example, If you defeat a Pikachu with a move like this:

Power 90, base attack 3, deal 30 damage

Pikachu originally had 15 HP
Pikachu now has -15 HP, then you get 15 extra EXP, so add that to 50 and you get 65 EXP total!
Understand? If not, go to the questions subforum and ask away!
Also, if you really want to, you can donate 500g to pick which pokemon you want to battle. PM me if you're going to do so.