Want to sell advertising space on my app... do I need a contract?

I want to sell advertising space on my app, and by this, I mean allow the company in question to put a page where the user can access their info and click a link to their site. The opening logo would also say something like the "Powered by Unity". It is a fitness-type app and I am currently looking at some smaller fitness companies.

Since I am sort of young (19) to be starting an app business, I do not know about this legal process. Would I have to get a contract drawn up for this process? I know I have to legally register a company before I do anything, but I also don't want to get into this endeavor if I am going to be spending thousands in legal fees to get a contract written up. I am currently in the process of submitting my information for a SMLLC.

Any advice from anyone who has done something similar would be very helpful!