Here's the deal: Anyone can get a sexually transmitted infection. Infections are spread through sexual activity. They don't care if you are gay, lesbian, bisexual, trans or straight.
In general, the highest risk activities are unprotected vaginal, oral or anal sex, but skin-to-skin contact can also pass infections. Some lesbians use dental dams for oral sex to help reduce the risk of infections like herpes. It is also important to be careful with any sex toys you use. Your best bet is to avoid sharing toys. If you must share, make sure to cover toys with a condom between partners.