As others have stated, individuals can't obtain CRB checks. Your employer has to submit the application (although, of course, you'll be required to sign the form to give your consent to the information being divulged).
The nearest thing that you can do yourself is to obtain a copy of your 'police record'. To do so, you have to submit a written application to your local police force, with proof of identity and a fee of �10. The police have up to 40 days to provide you with the information.
To find further information regarding how to obtain your police record, google this line;
"xxxxxxxxxx police" "subject access"
replacing xxxxxxxxxx with the name of your local police force (e.g. 'suffolk' or 'metropolitan').
Chris