In Pokémon XD, Pokémon trainers have their Pokémon battle each other. Since you will be doing this constantly throughout the game, read through here to learn the basics.
First off, each of your Pokémon know attacks, they may be able to directly damage the opponent's Pokémon, such as Tackle, or they may be moves that hinder the opponent's ability to fight, such as Sand Attack, which lowers the opponent's accuracy.
You can also use items you buy at Pokemarts during battle; they range from medicine to power-up items.
Also, you can switch your Pokémon with another Pokémon you have with you. This is useful for when you need to get one of your Pokémon out of the battle.
Moving on now, Pokémon are categorized by type. We have fire Pokémon, water Pokémon, electric Pokémon, and so forth. Pokémon attacks also are categorized by type. Types work in a Rock-Paper-Scizzers fashion; Fire beats Grass, Grass beats Water, and Water beats Fire. If you use an attack that is Super-Effective against the opponent's Pokémon's type, they receive double the damage you would have done to them. Also, in Pokémon XD, we have Shadow Pokémon that are Super-Effective against all types of Pokémon and are resistant to other Shadow Pokémon.
Also, be aware of what are called status effects. They cripple your Pokémon in some way. here is a list:
Poison. Drops a Pokémon's HP each turn.
Paralize. Slows a Pokémon and might prevent them from attacking.
Sleep. The Pokémon sleeps helplessly for two-to-five turns.
Burn. The Pokémon's attack stat is halved and they lose HP each turn.
Freeze. The Pokémon is frozen helplessly for one-to-six turns. Fire can thaw them out.