There are two possible reasons for your band saw to cut crooked.
1 is the set on one side of the saw blade has worn down and you need to change your blade, or
2 your band saw blade is not square to the bed of your band saw.

If the blade is brand new and it still cuts crooked then you need to square the blade to the bed. To check if the blade is square to the bed, lift the head of the saw up a couple of inches and take a square and set it on the bed. Slide the square up to the blade and to see if there is a gap between the edge of the square and the blade. Try to pick a spot on the blade where the tooth is straight or bent away from the square. If there is a gap you'll need to move both blade supports in or out until there is no gap between the edge of the square and the blade.
If you are unable get it squared yourself, contact the place you bought the saw from. They maybe willing to send a service tech out to do it for you. Some band saw blade makers are also willing to send a service tech out to correct any problems that you may have. Lennox is one such company that may send a service tech if you are using their blades.
Hope this helps you,
Hatter1