The most likely cause is that you're either collapsing at the front leg or not getting your action high enough. Also as said above you need to check your grip and make sure that you are nice and relaxed when bowling. Have someone watch your action (better still film it) so that they can talk you through it and detail when you release the ball and whether or not your collapsing. Having it on film will also allow you to make sure that other components (such as hand\wrist position etc) are ok.
Try to ensure that you're releasing the ball at the top of your delivery with a nice high action - this will help to improve control.
From here the next thing is to practice. I'd suggest marking up a wicket with tape on a good length (this will give you something visual to aim for). Work through your action as normal but concentrate on hitting the taped area. Don't worry about line at the start as this can be quickly added to the drill once you get the length sorted.
Providing you practice enough it should become second nature and you may wish to make the target area smaller (and move it around to just short of a good length, yorker length etc) to improve overall accuracy.
However, make sure that if you do practice the above drill that you so using your normal action - don't slow down\change things around in order to hit the mark.