Apply for your First Passport as an Adult
travel.state.gov
Information on how to apply for your first U.S. passport as an adult, or get a new passport if you do not qualify to renew a passport. You may need to apply in person if your most recent passport is damaged, or was lost or stolen.

Right now, you’re able to select the appropriate gender marker on your passport without a note from a medical professional (and a passport can be used in the vast majority of situations you’d use a birth certificate) but because the self select gender marker was done by executive action, it’s extremely likely to be reversed when Trump takes office.

Also, having one might be a good idea in general.