I'm trying to join the naval reserves (did 6 years active, honorable discharge) and and currently talking to MEPS. One of the questions I have to fill out is if i've ever been on meds/seen a psych. About a year after I got out I went to a mental health doc and was prescribed Zoloft, which I took for a few months. This was about a year ago. Will this disqualify me from joining the naval reserves? If not, will it affect my security clearance?

You will need to have documentation of the original diagnosis and the reason why you went off it. If you just decided to go off it or if your DR. took you off it. Also you may need another eval from a DR stating you no longer need the meds.

Depression can medical DQ you. As for your security clearance a mental health diagnosis by itself is not normally disqualifying (psychosis, alcohol/drug dependence untreated usually are)