Once the cards are dealt, the betting action starts with the player to the left of the big blind. He may either fold, call (by matching the big blind's initial bet) or raise (by increasing the big blind's bet).
If the pot is unraised by the time the action comes to the small blind, he may complete his blind by making the small blind up to the full bet for that round. If he opts not to complete his blind, his hand is folded. Of course, he may also raise by completing the small blind and adding another full bet for that round. If instead the pot has been raised by the time the action comes to him, the small blind must complete his blind to the full amount of the raise if he wants to remain in the hand.
The last player to make a betting decision on the first round is the big blind. If the pot is unraised when the action arrives he may either check, and remain in the pot for the flop, or raise. However, when he is facing a raise he may not check, he must either fold his hand, call the raise (completing his initial bet to the full amount of the raise), or reraise. If everyone else folds before the flop, including the small blind, then the big blind not only wins his blind bet back, but also collects the small blind's initial bet.
Once the first round of betting is completed, the small blind acts first on every subsequent round. If he is no longer in the hand then the first player still with cards to the left of the button must act first.