The Preflight check on an airplane is absolutely essential. Apparently there are people out there who scoff at performing a thorough preflight check. I am not one of them. Finding a problem on the ground is much preferred over finding a problem in the air. There's that old saying that takeoffs are optional, landings are mandatory and that alone should be a big enough incentive to make sure your plane works before you take off.
I could write an exhaustive article on preflighting, but the fine people of AOPA Flight Training have done it for me. Click here and you'll see an excellent general review of preflight procedures.
Of course, every airplane is different, and it's important to use the appropriate checklist for the airplane you will be flying.