Most of the replied here are correct with their methods.
Try to avoid at all cost using a lever to remount the tire because it usually ends up biting the tube and putting a hole in it.
Always start at the valve stem and work to the opposite side of the wheel. Mount one side first and then the other. When you get to that last section and it just won't budge, use a little water from your bottle and wet the braking surface and inner tire area and it usually makes the tire snap right on over the rim. Ever notice that they do this when mounting car tires?
Give it a try. As some replies mentioned, some tires are a real pain to get on and some are really simple.