The back crawl is different to most strokes because you cannot see where you are going. It is a good idea to count how many strokes it takes you to swim a length so you will know when you are getting close to the end of the pool. Try and swim with all of your body close to the surface of the water, almost like you are lying on your back in bed with your head on a pillow. Follow along with this swimming how-to video and learn how to do the backstroke if you are a beginner.
It is difficult to keep your body travelling in a straight line if you don't kick your legs. Use long fast kicks, making sure your legs are moving up and down. Keep your knees underwater and bent a little, and your toes should make a small splash when you kick.
The arms provide the power in back crawl, making a circling action as they move in and out of the water. You start by putting one arm in the water in a straight line above your shoulder. Once your hand is in the water it should push down and towards your feet. Bend your elbow slightly and pull your arm by your side to your thigh. Keep pushing your hand towards your feet until your elbow is straight. Then lift it out of the water, back to its original position and repeat the motion again. Keep the arm straight all of the time it is out of the water. The left and right arms do the same movement, but not at the same time. One should come out of the water at about the same time as the other enters it.