How often do you need to get std testing if you are sexually active? what counts as "sexually active"?

It varies a lot: "Sexually active" can mean anything from mutually monogamous with one uninfected partner to multiple casual sex partners (perhaps without condoms) every week. The first doesn't need STD testing at all, the second as often as once a month. Sex of partners is important -- higher risk for gay men than straight men and women. For most hetersexual singles, every 6-12 months probably is about right.