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.
You are venturing into a business partnership with your best friend, your spouse, or your cousin. If you think you are safe from all the business conflicts that arise in most start up ventures because you know your partner very well, think again. Most conflicts and legal contests are because of this ignorant belief. You have to protect not only yourself but also your partner. Hence it is best for you to draw up a partnership agreement template before you decide to commit yourself.