You can't get a CRB check on yourself.
However, you can effectively do the same thing (for most purposes) by getting hold of a copy of your police record. (Many employers will accept this in place of a CRB check, although they might still require you to allow them to apply for a 'proper' CRB check once you start employment).
To obtain a copy of the data, which the police have about you, you simply need to write to your local police force, provide proof of identity and pay a fee of �10. The police must provide the data (or a document stating that no data is held) within 40 days. Your local police force's website probably has further information but the procedure is a standard one, so this might help:
http://www.westyorkshire.police.uk/section-ite m.asp?sid=77&iid=3984
Chris