Your OBGYN is your partner, not your enemy
The very first thing you need to know is that your OBGYN is your partner, not your enemy. If you have a good OBGYN who you are comfortable talking to, you can get an awful lot of reassuring advice. It can be pretty awkward to talk about menstrual pain, premenstrual syndrome and birth control to your mom or even a friend.
Your OBGYN will be able to do more than just listen to you she can present real solutions. Perhaps you do not have any questions about anything gynecological, and are just going for a preventative checkup. But if you are anything like most other girls and women, you will have a ton to discuss with your gynecologist. Perhaps you've noticed abdominal pain, suffer from heavy periods, of have a yeast infection. Or perhaps you want to discuss birth control options, or STIs, or pregnancy.
Whatever the reason you are going to an OBGYN for the first time, it is important that you like your doctor enough to feel comfortable with her. (I say her, because I think most girls are more comfortable with a woman I know I am!) If you don't like your OBGYN, they do not treat you with respect, or you are worried they may talk to your parents about your appointment (it can happen, though it is not even legal), then see another doctor. Your doctor should be on your team.
Your first appointment
Your first OBGYN appointment is a great time to get to know your gynecologist. The American College of Obstetricians and Gynecologists recommends every teen to have their first routine gynecological visit between the ages of 12 and 15. You will probably talk about your medical history, and the gynecologists will ask questions about your menstrual cycle, whether you are sexually active, and about contraceptives. It is standard practice for gynecologists to suggest an internal exam once you are 16, and a PAP smear once you 21. If you are sexually active before that time, internal exams are also recommended.
Testing for sexually transmitted infections is very important to sexually active women of any age, and regardless of whether they use contraceptives or are monogamous. If you have had sex, getting tested will give you peace of mind. (It's fair to say that you should look for another OBGYN if they make you feel bad for "not waiting for marriage" or anything similar. They are doctors, not the moral police.)
The internal exam
Let's face it, that's what we are all most intimidated by. It is pretty frightening to lie on that exam table with your legs in the air in stirrups, perhaps while you are wearing a gown. It's nothing new to your OBGYN, but you should feel absolutely free to let her know that you are frightened, and to request that she be very gentle.
A tool called a speculum is inserted. These are most often made of stainless steel and can be really cold. You can ask if they can warm it up first, though. Your OBGYN should go along with that. Some girls find internal exams very painful, but it should be over very quickly and not cause any discomfort (besides the psychological discomfort, perhaps!).
Some teens' mothers come to their daughter's OBGYN appointments and even want to come into the exam room. This is absolutely fine if it is what you really want. Most teens prefer a bit more privacy and want the opportunity to chat with their own doctor in privacy. I recommend you have a different OBGYN to your mom if possible.
Your mom can wait outside in the waiting room, or even outside in the car, while you are being examined. If your mother has not made an OBGYN appointment for you yet, you may like to get in there first and get together with some friends. If you go for your first appointment with a few close friends, it is all less scary.
You can ask your mom to help you book the appointment, sort out the insurance details, and to tell you a bit about her experience with OBGYN checkups. She'll be really proud that you have been so responsible, and she will not be in your way. You may also find this article useful: Do you need OBGYN checkups when you are not trying to get pregnant?