It seems unusual to receive a call My Mid's candidate number came by letter.
Anyway, just log in and fill out all the required information. Unless it's changed, there will be sections for academics, athletics and ECAs, and so forth. You just pick a section, open the window, and fill out page after page. It will be much easier if you have a printout of your NASS application (if you applied), or your resume. You will also need to supply the names and email addresses of your 11th grade English and math teachers. USNA will then send them some type of recommendation form to fill in on-line. You will also need to schedule an interview with your BGO. Your assigned BGO's contact info should be listed when you log in (I think).