You can't. If you don't want your mom finding out and you have very little or no money, go to your nearest Planned Parenthood. You're allowed to appear anonymously, so no ID required, but do say that you're 18, because if you say you're under 18, a minor, they can't knowingly treat a minor without parental consent. You will be examined, which will check you for all STDs, and then you will be treated for little or no cost. As for the STD test results you won't get back right away, usually about a week, they give you an anonymous ID number to use and a phone number to call with that ID number to get the results and find out if you need to be treated for anything else.
Just Google "Planned Parenthood." If you have a university near you, chances are there's a Planned Parenthood right nearby. Almost all of Planned Parenthood's business, which is non-profit, is examining and treating young people for STDs, even when they have insurance through their parents because they don't want to go to a regular doctor and use their insurance because it would mean their parents getting an explanation of benefits (EOB) from the insurance company showing what tests were run, what treatments were given, and what their respective costs were.