The best method of protection against sexually transmitted infections is by using a male condom or a female dam every time you have sex. Click here if you are unsure on how to use a male condom or here for how to use an internal female condom. Barrier methods shouldn't be limited to penetrative sex and should be used when engaging in oral sex also.
Limit the sharing of sexual aids such as toys. If you do share them, make sure to clean them thoroughly between each use and between each partner. Make sure to cover sex toys with a condom before each use.