If you would like more information on rimming, we suggest you read What Is Rimming? on the Go Ask Alice website. You will find the definition of rimming, information on how to protect yourself from the transmission of STIs when engaging in this kind of oral-anal contact, and how to prevent bacteria from spreading from the anus (especially to the vagina which could cause vaginal infections).
Since you mentioned that you had sex for the first time, FX wants to stress the importance of protecting yourself from unwanted pregnancies and the transmission of STIs. Teens are generally recommended to use condoms as well as a back-up method of birth control such as the pill when sexually active. In addition, you didn’t say why you are “positive” your boyfriend does not have any STIs given that he has been sexually active prior to being with you. People can carry various types of STIs without being symptomatic, so unless he has been tested he cannot be 100% sure he is in the clear. If he has not had any testing done, and given that you have been sexually active with one another, FX recommends that you both speak to your doctors about whether testing is advised and about the best birth control methods for the two of you to be using.
If you don't have a doctor and live in northern New Jersey, you can call the Adolescent/Young Adult Center for Health at 973-971-5199 for an appointment with an adolescent medicine specialist or contact your local teen health center or Planned Parenthood. You can also contact your insurance company for a list of in-network providers.