What exactly should be in this type of contract? Well, firstly you need to be aware of any laws in your country regarding employing people. It is likely these laws will override your agreement so it makes sense to line them up so they match. Then you need to put the basics in there. This might include hours of work, what their role is and how much they are going to be paid. After this, it is a matter of adding some other clauses to protect both you and the employee. This could be to do with dismissal and other situations that might arise.
You can pay a lawyer to draw up this agreement if you are a large company that has a big enough budget to cover the costs, but this is not necessary as you can do it for yourself. You can of course write out your own agreement using one of the many different templates that are available for download online. Both of these documents are completely binding in a court of law, the difference is the amount of money you are going to have to spend. Each of them must be signed by both parties and in many states notarized in order for them to be legally binding.