Hello,
I used to feel the same way cos this is also my first time, but I think the next step is you have to wait for the I-20, the document you have to use when you apply for VISA, in the meanwhile, you should try to find apartment, find info. about the dept. you are going to study ( about the faculty, the study env., etc.), make the final decision whether you 'll accept their offer and then, you should book an air ticket, when you got the VISA, everything is done ( I mean you could fly to USA), about the course registration, I think the dept. will tell you more when you accept their offer.
Good Luck:D