What a trainer challenges you to a battle, you can either accept or decline, depending on the state of your pokémon. If you accept, the challenger will attack second, unless they have an attack like Quick Attack.

If you start a battle, after every post you have to put up your pokémon's stats. You can use your own format, but here's a coded one for your convenience.

Code:

[b]NAME OF POKÉMON[/b] [b]HP;[/b] ?/? [b]STATUS;[/b]

And it'll look like this;

CHARMENDER
HP; 17/23
STATUS; Burned

For example.

When calculating damage that your pokémon has suffered, first of all take in the type of that particular pokémon. For example, Water attacks are twice as effective against Fire types, while Fire attacks are half as effective against Water.

Then take into consideration the Level of your opponent. Sure, on the same level a Fire pokémon would be disadvantaged against Water pokémon, but if a Fire pokémon were twice the level of the Water one, then the Water one should be at a disadvantage.

Finally, think of the effects the certain attack has. For example, Ember can cause a burn. In this game, there is 50% chance that the effect will work. If you are unsure, flip a coin. If an attack (eg, Bite) causes flinching, it is also 50%.

And lastly, if there is a certain attack (Counter, for example) that will deal double the amount of damage that the user has recieved, count only the latest attack, not the whole battle.

And finally, I will tell you when you can level up. Pokémon who lost can also gain experience, but only half that of what the victor got.