I guess that the spring isn't hold the sprocket well. The best is to bring the bike to shop to repair it.
But if you want to repair it all alone, first get your rear wheel off, get the sprocket off and check the 3 teeth inside the sprocket - they have to be perfect half-circles. If they aren't, you should replace it.
The spring must touch itself on its' ends. If it's not touching, put it pliers that way that the ends of spring will be out of pliers and squeeze it more than needed, so the ends will come round one on other, then turn the spring this way that the ends will be inside the pliers and squeeze it again. Check the spring, that it tight enough.
Put the sprocket on back. To return the spring you should use a screwdriver or something.
If the problem still exists (though it shouldn't), it's inside the wheel, and there is more complicated.