Writing a personal ad shouldn't be as hard as you might imagine it is. This page includes a couple of examples to give you an idea of what elements of a personals ads you might include with your own. The best thing to do is join a dating service, and read a lot of profiles. As you read take note of what works, what doesn't and use that knowledge when writing your own.
When creating your profile, get a pad and pen, go away from the computer, make yourself a cup of coffee or tea and find a quiet place where you can spend 15-30 minutes thinking. It shouldn't take long - start by jotting down things you like, qualities you posess and what you're proud of having accomplished.
Also make notes of the positive attributes of any past partners you have had. Mention interests and activities you enjoy and any new hobby you might be taking up. Once you have a list, use the examples below to give you an idea of a basic profile. Of course, write it in your own style, but don't overdescribe yourself - you want it to be short enough so any potential matches are intrigued enough to want to get to know you more and to make your personality heard. Just remember, this isn't a life story you're writing.