Relax, be cheery, know your position and know the office as best you can before you walk in. Try to keep on point on what you can do for them! Answer honestly, and fully... make eye contact, smile...
And what I tell my husband is, don't ask any questions about money, benefits, or otherwise until the end of the interview when they ask if you have any questions for them. You want them to want you first!
Dress neatly, office attire, be a few minutes early, try to just be relaxed and friendly. Before you go take some time to consider the job and the office itself and come up with a few good questions to show you really are interested in what you would be doing there. Act confident! Even if you have to fake it! Speak clearly and answer questions honestly and to the best of your ability. I know all this sounds like plain common sense, and it is, but you would be surprised the things I've seen and heard in interviews!
Also, consider that this job may be a very good "foot in the door" to move up to higher paying jobs in the medical field. I did not go to college but I worked for 21 years in the medical field and there are many, many good paying opportunities there.