Individuals can't apply for CRB checks (including on themselves). However, you can show that you have no criminal convictions (or no relevant convictions) by obtaining a copy of your police record. To do this, you should apply in writing to your local police HQ, enclosing a fee of �10. The police may require you to produce evidence of ID. They have 40 days within which to provide you with a copy of your police record (which may, of course, simply be a statement that you have no criminal record).
Chris