Find a doctor or clinic.
My gyno tests me every time I go, most of them do. Sometimes you have to request for more uncommon STD tests, but the majority are generally covered. If you haven’t started seeing a gynecologist yet, ask your regular doctor to test you. You might be embarrassed, but seriously, they don’t care! They’d rather you ask instead of waiting until it’s too late. This is part of their job and with patient-doctor confidentiality, you don’t need to worry about mom and dad finding out.
You can also go to a clinic like Planned Parenthood and get tested. You can find clinics nearest to you on their website. At most clinics, parental permission for minors is not required.