Ideally, a lawyer would draft this for you individually, which will cost you a few hundred dollars. But if you must use a contract you've downloaded from the internet, at least have a lawyer look over it for you once you've filled in the blanks. It'll be quick, painless, at little cost to you, and you can reuse it for your future advertisers. Most solo attorneys I know wouldn't charge you much at all.
Marco Santori is a lawyer in New York City who works with small and medium-sized businesses. However, he is not your lawyer and this post is not legal advice. If you need legal advice, PM him, or check out http://www.marcosantori.com