Nothing to be afraid of, assuming of course that you've checked their credentials.
As for duration of treatment, I have a vastly different opinion to mrfrench's (please note though that he, as I, speak from our own personal experiences) and despite you being nervous about going, I sincerely hope you take my opinion into account as well and make up your own mind.
It largely, largely depends on what the issue is. If it's something that's taken a long time for something to surface, usually several years (subluxation of the spine for example), due to poor posture, work, habits, sports or whatever, then obviously it'll take equal, if not more time for the issue to be reversed and corrected. "A couple of visits" and you're done is, and again I speak from personal experience, a lazy, poorly educated chiropractor who may be more concerned about your quick buck than your long-term health.
You go in, they crack your neck, back, whatever and you feel good for a few days, maybe even months, but you're nowhere near long-term better and healthy. The issue will come back tenfold because it'll just worsen with time without it being taken care of properly.
A couple of visits and you're taken care of would be the equivalent of eating McDonalds everyday for 5 years and then going to the gym twice and looking like Ryan Reynolds in Blade Trinity.
A good chiropractor will take x-rays of you, compare your results to normal, healthy alignment, explain to you what's wrong, make an educated guess at how it happened and have you fill in the gaps. Then tell you what needs to be done to reverse your condition and ultimately leave it up to you if you want to commit to it.
Once you commit to it, you'll come in for whatever procedures are necessary and he'll also give you suggestions on what you can do at home or on your own time to help your situation. Every 6 months or so a good chiropractor will take more x-rays to review your progress and as you get better you'll have to go in less and less.
One thing to be careful of though - if your chiropractor suggests that you use heat instead of an ice pack, run for the hills.
This of course depends on what's wrong with you.