In order to start betting in Hold'em, forced bets (known as blinds) are made by the two players immediately clockwise from the dealer button. The person immediately clockwise from the dealer has the small blind, and the next player clockwise has the big blind. Making blind bets is known as posting and this is done before any cards are dealt.
The size of the bets are determined by the limits of the game that you're playing and the small blind is nearly always half of the big blind. So a $2/$4 Limit Hold'em game has a small blind of $1 and a big blind of $2.
Blinds are forced bets. The players in these positions must make these bets or they aren't dealt cards in the hand. These blinds, in turn, force betting action on the table after everyone has been dealt their hole cards.
At a casino, when you first sit down at a Hold'em table, the rules vary as to whether you have to post blinds (even if you're out of the normal blind positions for that hand) in order to be dealt a hand.
In Las Vegas, you're dealt a hand as soon as you sit down and have shown that you meet the table's minimum buy-in. You're not required to post a blind in order to get hole cards. Conversely, in most California card rooms, you're required to post a big blind in order to get your starting hand.
In cases where you're required to post a big blind before you're dealt cards, you're mildly better off just waiting until it would normally be your turn to get the big blind anyway, rather than jumping straight into the hand. Waiting like this keeps you from making an extra forced bet and gives an added bonus of being able to case the players at the table while you aren't actually playing. Dealers are used to this behavior and will probably ask you if you want to sit out (that is, wait until it's your turn to post the big blind).
How soon you post is a fine point, though, that doesn't really make that much difference. If you're itchin' to play, or if you have a very limited amount of time to play, go ahead and jump in. The dealer will tell you whether you're required to post a big blind.