To be a full-time graduate student, you need at least 9 credit hours. This can be achieved with two core courses and one more. Luckily, there’s a prescribed BIOS core course sequence that often includes two core courses, so you’ll just need to find your third course. This can either be a BIOS elective that you’ll have to take eventually (for instance, BIOS 511 or any of the 700 levels), or one of the general requirements (for instance, EPID).
The course requirements may be found in the Academic Information Manual (AIM) and vary depending on your degree program. Note that things have changed over the past couple of years (for instance, you won’t be able to register for seminar your first year). Your Big Buddies might not be aware of these changes, so always check with the student services managers and the AIM.