1) It all depends on how you want to join the Navy. I went to a community college first, then transferred to a University. I applied for OCS after I graduated. You can also attend a University that has a Navy ROTC program and apply for it. To my understanding, you get help paying for college and you don't attend OCS. If I had to do it all over again, I would do ROTC because I would earn my commission directly after graduation instead of, in my case, 2 1/2 years later.
2) I've been told by my recruiter that a 3.4 was competitive for the Supply Corps. (I have a 3.6) Crypto, intel, ect.... need to be higher 3.6 and up. Pilot, I'm not sure. I've seen some 3.2's but they had high ASTB scores.
Hope that helped.