I need hel understanding Security clearances?

The recruiter would have already told you if you have too much debt probably. They look at what your debt to income ratio will be. If you can afford your payments it won't be a problem. As a lower rank you won't be making a lot of money so they won't let you come into the service if it looks like you can't pay it. The issue for security clearances is if you have so much debt you'd be liable to being bribed for clified information if you had access. You won't have a security clearance yet when you go to swear in. You may need one for AIT and in that case they'll grant a temporary one.