Like any other skill the only real way to improve is to practice.
Right, but that's not really telling HOW... sometime when you get a few free minutes, practice just kicking the ball as far as possible (with relative accuracy) in an empty field or somewhere similar. This will help improve the distance of your shot. If you can get the ball about 30 feet easily, it's time to work on technique. The best shots are hit with the laces of the shoe, usually at an angle. When kicking, firmly plant the foot you are not kicking with next to the ball, in the direction you want it to go. This is very important; the direction of your planted foot is the direction the ball will go, if you are balanced right. Work on getting the top of your toes under the ball, (don't kick with the tips of them) so to get some lift on the ball. (A weak shot hit on the ground isn't going to go in; hitting it in the air is your best choice.) If you are having trouble doing this, you can practice putting your foot in the right position without kicking, to get a feel for where your foot should be. Then aim for a corner of the goal, and go for it!