My 2p's worth -
1. Learn to edit.
2. Put down everything that you've ever done; guys your age haven't done much yet, so you have to make your application stand out.
3. Do NOT put any mention of things that you might do or intend doing, unless you have been accepted on such-and-such a course and have yet to start it.
4. Don't commit yourself to a DofE just to make it look good on paper, but don't be put off it if you really want to do it for yourself.