Okay, well, a few things.
When you make a deck, you usually want to have 15-25 Pokemon, generally, 15-25 Trainers, and the rest should be energies. You are allowed 4 Special energies (IE, double colorless) and 1 Arc Spec card (IE, Golden Potion).
However, some official league hosts (the person who controls leagues and tournaments and whatnot) might tell you different numbers. My guy told me 20-26 Pokemon, 20-26 energy, and the rest should be trainers.
I have learned that is not correct.
As for playing, you'll need a 60 card deck- no more, no less.
Most people, like myself and trachy, would assume you're using the current format.
That means Black-White through Boundaries Crossed. Anything not released is obviously not going to show up in official gameplay, and anything before BW is considered illegal.
Something called Special Conditions will show up every once-and-a-while.
That would be, poison, burn, paralyzed, confused, and asleep.
When a Pokemon is poisoned, place a poison counter on it. (Most players dont because our memory isnt crippled yet) This means that that Pokemon is poisoned and will take 10 damage in-between turns (both yours and your opponents).
10 damage is standard poison, but a lot of cards today will have you doing 20, to up to 40 damage in-between, if not more. Poison will disappear when you are either KO, or you retreat to the bench.
If your Pokemon becomes burned, put a burn counter on it (once again, not needed, but useful). When a Pokemon is burned, in-between turns (both yours and your opponent) flip a coin. If it is heads, you don't get burn damage for that turn. However, if you get tails, you take 20 damage. Like poison, different Pokemon will effect how much damage it takes. Burn will disappear when you are either KO, or you retreat to the bench.
When a Pokemon is paralyzed, turn it to the right, sideways (I actually recommend doing this, but once again, you dont have to. Just remember that memory serves, though some juniors might yell at you. :P ) When a Pokemon is paralyzed, it cannot attack, retreat, etc. At the end of the turn, a paralyzed Pokemon will become un-paralyzed.
When a Pokemon is confused, turn it upside down (I actually recommend doing this, but once again, you dont have to). When a Pokemon is Confused, you may still attack, but you must flip a coin. If you get heads, your attack will work, but if you get tails, you take 30 damage and your turn ends there. When a Pokemon is confused, it may not retreat to the bench, though it can still be Switched out to the bench, via trainer card. Confusion will disappear when you are either KO, or you use a trainer card to retreat you to the bench.
When a Pokemon is asleep, turn it to the left (I actually recommend doing this, but once again, you dont have to). When a Pokemon is confused, you cannot attack or retreat. Once between turns, you may flip a coin. If you get heads, you wake up, but if you get tails, you're still asleep. Asleep will disappear when you are either KO, flip a coin and get heads, or you use a trainer card to retreat to the bench.
There is 3 ways to win a battle.
1). You draw all of your 6 prize cards. This is the most common way.
2). Your opponent cannot draw a card at the beginning of their turn. This will usually take a longer time, or they're running a Durant deck.
3). Your opponent does not have a Pokemon on the field.
It is critical for a Pokemon to be battling, so its important to have strong lines that can build up quickly and attack fast and hit hard. When you KO a Pokemon, you take 1 Prize card, but when you KO an ex, you take 2 prize cards.
If you're confused about anything else, please tell me so I can answer it. :P