If you are asking those questions, you have a lot of work ahead of you.

Go to: Baseops Military Aviation Forums for info about the AF + Guard
Go to: www.airwarriors.com for info about Navy/USMC/USCG

Please do yourself a favor, and spend a week or two reading and lurking before you ask any questions - they've all been asked before, and with a little research, you can find the answers.

But to answer your question specifically, each service has several commissioning programs. Once you pick one, you will have to sign a contract with the recruiter. The answer lies in the details of that specific contract.

I had an aviation contract with the USMC. Which meant that as long as I didn't poke my eye out going through OCS, and then through 6 months of TBS - they would send me to flight school. But I wasn't guaranteed anything except the opportunity to train. After primary training, everybody branches off to either helicopters, jets, or props - and it is based on performance + needs of the service, with a heavy emphasis on the needs of the service.

Some people go into a service with an open contract, which means they have to compete for their job. For example, when Navy ROTC students graduate, they will select pilot, surface warefare officer, submarine, etc...

As far as I know, the only way to be guaranteed a specific airframe is through the Air National Guard. You could walk in, get hired, and go to flight school with a guaranteed spot for the aircraft you want to fly. However; chances are high that some other college kids with a 3.1 are applying, and want the same deal. Right now you need to focus on the grades, and then do some research on the side.