How can i prevent an std after i have unprotected sex?

Too late: After sex, is exactly the wrong time to think about pretection from getting an std. The only sure way to not get an std is to not have sex. That includes oral and anal sex which also have risks. Use protection always and have sex with limited partners and have each get tested often to know their status.

Can't prevent: After the fact...Hopefully you come out unscathed and use better judgement next time.