Like Vic say's being nervous is a totally normal reaction. It's normal because you are going into a group of people you don't know and possibly sharing very personal details of your life. It is normal because I'm sure you are going through some inner struggle about using and this is a positive step towards NOT using. Part of your brain is going to react negatively towards that.
What I can tell you, from my personal experience, is that nobody will preasure you to share any more than you are comfortable with. If you don't want to share at all, that is ok too. Most likely people will approach you and welcome you because they don't reconize you. Some may offer their phone numbers, so you will have someone to call if you need support. At some point in the meeting they may ask any "newcommers" to introduce themselves. You dont' have to do this if you don't want to, but I would recommend it. All you have to do is give your first name and most people follow it with "and I'm an addict" but if you don't want to, you don't even have to do that.
The most important thing to remember is that almost every single person that you meet has felt exactly the same as you feel the first time they come there. Very few people, are exicted and happy about going to their first meeting, though some are. As others have said, what you will find is a group of people similar to the motley crew you find here!! They are there to help and be helped. Take care.