Rates at MEPS

I just found out today that my Air Force recruiter tried to get me to sign for a open contract (meaning that I would not pick my job, but my job would be picked for me). I personally do not like the sound of this and don't want to be stuck with a job that I do not like. I have read that I will pick my naval rate at MEPS, If I decide to enlist. Is this true? Will I have a rate reserved for me after I leave MEPS. Will I be able to choose what my rate is? ( if my ASVAB score, and medical review clears me for it of course). Also do Naval Officers choose their jobs also or is that up to the needs of the Navy?