Do I need a job offer before I can enroll?

Not to enroll at most schools, no. You'll need a real job when you apply for CPT itself - the school reviews your offer letter and duties, and most use a CPT Agreement your employer signs. A few partners are stricter (one needs a job or internship before you start, and some require you to stay employed, with a short grace period if you lose the job), so I'll flag those on your shortlist. Do you have a job offer in hand?