Security Clearance Lawyer
If you have been requested to apply for security clearance for a job that you have been accepted for then it would be a good thing to first of all have a chat with a security clearance lawyer.
The worse thing you can possibley do is go ahead and fill in those forms, get them wrong and have your application turned down. It is time consuming and can be expensive to have a security clearance application decision overturned. Not to mention the fact that it may jeopardise your chances of getting that job.
There is plenty of free information around that you can read and research. This information will give you enough to go on and enough to start preparing your forms for your application. You could also check out a security clearance lawyer and find out the ones who offer a free initial consultation.
You can then decide whether or not you need the services of an expert to help.