Basics of the Battle
There are two basic reasons for a Pokémon to battle. One is for sport. You can battle another trainer in a friendly competition. Your Pokémon do the fighting, but you decide which Pokémon and which techniques, or attacks, to use. Battles for sport are important because your Pokémon gains valuable experience points, learns new techniques, sharpens old techniques, and evolves to the next level much more quickly. Plus, it's fun! Pokémon like to play.
The second good reason to battle is the capture Wild Pokémon. Wild Pokémon have no training, and no owners. They can usually be found in long grass, dungeons, or water (you'll need Fishing Rods to catch these), and sometimes in amusement parks like the Safari Zone or Coin Exchange. Battle is one of the main ways to collect Pokémon. You can also buy them in some stores, or trade for them with friends. Some trainers may even want to give away their Pokémon! Other trainers' Pokémon are off limits, however. You can't capture these Pokémon even if you win a competition.
Choose the best Pokémon for the job
Each trainer can carry around a team of up to six Pokémon at a time. For each new situation you encounter, you will need to decide which six Pokémon it is most advantageous to carry with you. The rest are either kept with the Professor who gave you your Pokédex, or in a high-tech computer storage system. You can use as many of the six Pokémon you are carrying in a single battle as you want, but the limit to use simultaneously is three. Feel free to switch Pokémon at an time, especially if the Pokémon who's fighting looks like it's taking a beating. If you win a battle, each Pokémon that fought gaints experience.
The longer you train, the more you'll learn about keeping a team with a variety of techniques and strengths. You'll also learn which Pokémon are best suited to which battles.
In the Gym or on the road, other trainers are eager to show off their prized Pokémon and techniques. If a trainer challenges you to a friendly competition, you must accept. It's rude to refuse. Don't think you'll get off easy, either. You team will have to face, and hopefully defeat each and every Pokémon on the other trainers team. You win when your Pokémon have defeated all of the other trainer's Pokémon. A Pokémon is defeated when it gets so weak that it faints.
So what's in it for you? In addition to all the experience your Poké friends gain, you earn a cash prize for every competition you win. It's money you'll need to buy valuable supplies.
The Poké Ball
To capture a Pokémon, you'll need a Poké Ball (Japanese: モンスターボール Monster Ball). Poké Balls are what you carry your Pokémon in while they are learning and training. Only one creature fits in each ball, so you'll have to have a good supply on hand. Most Pokémon will only leave their Poké Balls when you command them to, though all cases are unique. Some may complain about being in a Poké Ball, but only Pikachu will refuse to get in. There are a total of 26 various types of Poké Ball. Some Pokémon, like Snorlax or Golem, are just too strong and too experience to be caught by an ordinary Poké Ball. As your adventure continues, you'll be allowed to purchase Great Ball and Ultra Balls for Pokémon like these. Some Balls, such as the Safari Ball, are only able to be used at certain events. The Master Ball is only given out to experience trainers and Pokémon Researchers. It is the strongest Ball, able to catch any Pokémon. Some Balls can only be made from Apricorns (Japanese: ぼんぐりのみ Bonguri Fruit), a special fruit that is native to Johto and Hora, though some are growing in Kanto.
Pokémon rarely die in battle. They normally get very weak and may even faint. The goal in any battle with a wild Pokémon to get its energy down low enough to capture it.
Once a Pokémon has been weakened, it's Poké Ball time. Throw a Ball at it. And remember, there are no guarantees. A weakened Pokémon may still break out of the Ball. If it doesn't, it's yours to train and will, hopefully, be loyal only to you.