Celexa is an SSRI, all of which are more or less the same. There isn't anything to be afraid of. Millions of people take SSRIs without trouble.
You won't notice an effect just from taking one pill, like you would taking aspirin for a headache. It's a gradual thing. It might make you feel kind of sick for the first week or two, but that passes. It may take a little while longer to start feeling better. The effects aren't easy to describe, but you'll notice feeling differently. It isn't a sudden night-and-day change, and it isn't a "happy pill", but it does make you feel better.
Any side effects that stick around after the first few weeks are usually so mild as to be overshadowed by the positive effect. It should go without saying that if you experience any effect that's unacceptably unacceptably awful, you should talk to your doctor about switching to something else. Only something like 3% of people ever quit over side effects.
They definitely aren't addicting, but abruptly quitting after you've been taking them for months can cause some unpleasant side effects. Nothing terrible, but it's worth mentioning so that if you do decide to quit, you should talk to your doctor rather than just stop taking them on your own.