Prevention is better that Cure.
The best one is,
Never have sex with anyone. But then again we are just humans and we need sex for satisfaction. So this might not be possible.
Always have yourself tested. HIV testing is free, some STD tests may be expensive.
Always practice safe sex. Use condoms at any times. If you engage in oral sex make sure that you have no wounds or lacerations in your mouth as they can be the entry point of the bacteria and viruses from your partner's genitals.
Never engage sex with just anyone you meet in the club or in places where prostitution is prevalent. Always check the background of the person.
When you are in doubt that the person you are about to have sex with is safe, abort the plan, go to the door, go home and play with your self instead.