First... you can't just walk into a community college and sign up for classes. You have to apply and be accepted first. When you walk in tomorrow, all you'll get is an application and, if you need them, forms for financial assistance. You won't even see an adviser until you're accepted and ready to register.
Second... you usually can't obtain a certification in anything with only 15 credits. That's only about 3 courses. You'll have to complete a radiography program, which generally consists of around 60 credits.
Third... the classes you take are dictated by your program of study. All you adviser will do is tell you which core classes are required, and which non-core classes you may choose to fulfill any general education requirements for your program. They're not there to sell you on "extra" classes.