The most important part of your ad. Remember if you don't catch your reader attention you just loose him and you won't make a sale. So your header must be short and attractive. If you are not sure browse the Free Ads on line or your local newspaper. I let you answer this one by yourself.
Body Here come the easy part.
Remember you already get attention from your reader. Now he wants to know what is good for him in your offer. So you have to get him interested in knowing more. Write down some kind of fact in a few short phrase. Keep the details for your sale letter.