You should
The classes are actually designed so you are in control of your own pace and resistance. You can go as hard or as easy as you like. So even if your fitness isn't great yet you can still get in and give it a go.
The instructors will help you with bike set up etc, just get there a little early to ask them.
You'll find the atmosphere of the class will tend to motivate you to work harder anyway. Spin is great because no matter fit you get you can always find another gear to crank it up.
Oh, and give it at LEAST three classes before you decide you don't (or do :P) like it. Generally everyone hates the first one and by the end of the third is addicted lol.
Have fun!