I'm a lesbian, can I catch an std?

Yes. Anyone can be exposed to a sexually transmitted infection. Women who partner with women can still be exposed to viruses like hpv, HSV and some bacterial infections as well. To minimize exposure, was and disinfect any toys and use barriers like gloves, dental dams, fingercots or condoms with your partner(s).
YES. Anyone who engages in sexual contact with others involving skin-to-skin or skin-to-secretions type contact is at risk for stds. That involves sex with any type of person and involving any type of body part involving skin and/or body secretions.