If the door sticks because it has been exposed to humidity, there are two things you can do. First, simply wait until the weather is drier and the door returns to normal. If the problem is one you need to solve now, see where the door is sticking. Remove the door and gently plane the spot that is causing the problem. Be sure than you do not overdo it, because the door will eventually return to normal when there is less humidity, and you do not want unsightly gaps to appear.
Poorly adjusted hinges could be the culprit. Look at the hinges, both on the door and the frame, to see if they are loose. If possible, tighten them. You may find that the screw will not tighten because its hole is enlarged. You can replace the screw with a longer one, making sure that its head is the same size as the one you are replacing.
If loose screws are not the problem, the hinges themselves may need adjustment. Close the door and determine where the door is sticking. In most cases, a gap will appear right across from where the door is binding. Reopen the door, and hammer in a small shim between the hinge and the doorframe at the opposite end of the problem, if it is along the side of the door. If the door is sticking at the top or the bottom, Choose the hinge farthest away from the problem area.
If the door sticks because its frame is misaligned, you have a few options. Try setting a piece of wood against the door-frame where it is sticking and give it a good whack with a hammer. The frame might just move enough to solve the problem. If that does not work, you might consider using a plane where the door is uneven. Be careful not to plane off too much or the door will be uneven.