You mean you want to PRODUCE & direct YOUR OWN film or just direct someone else's? Also, what do you mean by in charge of the contracts and such? A lawyer can write up an agreement between you and the producer, or between you and all the other crew and cast members if you're also the producer. If you mean sign a contract with the producer on your behalf, legally and technically you can sign the contract yourself. Also, what exactly do you plan on putting in the contracts?
HOWEVER, you seem to misunderstand how these things work. Some of it is common misconceptions, some ignorance when it comes to the movie-making business.
First of all, directors simply direct. They're hired by the producer, who also hires the rest of the crew and cast members. The producer is also the one buying or opining (google the term) the script, or they commission a writer to write the script for them from scratch, and they work with the writer(s) on the rewrites. The producer is the boss. They fund the movie, either from their own pocket or they get the funding somehow. They make all the production decisions. Basically, it's THEIR movie, as opposed to the common misconception, not the director's unless the director happens to be the producer as well.
Now. If you want to make your own short film, by that act as the producer (and the director?), I suggest that you focus on practicing and learning for now. Directing and producing is not that simple, far from it. It takes skill and knowledge (of the craft and of the biz) which takes years to gain. I wouldn't focus on things like contracts and agreements, it makes little sense. I'd just get together with friends or collaborate with other aspiring (filmmakers, actors, writers, editors...). Have fun learning from the experience. Then do it again and again. Improve. I also wouldn't get into financials. Since you're just starting out, chances are your movie will not be that great, so I wouldn't invest that much money on it, if any.
If what you want is to direct a short film for an (indie) production company, I'm sorry but that's not going to happen at this point of your life. There's actual money invested in producing films, no one will hand the wheel to an unknown minor with no experience or prior success. Production companies also usually use their own crew members and not someone from outside, or they hire someone they know and possibly worked with before. So again, if you want to direct one day, start by getting some experience. Learn about the craft AND the business. You need to understand that even an indie is still a business. You invest (risk) money and time in the hopes that it will later on pay off, whether it's by making money to make more films, by harvesting attention and fans, or by winning awards at film festivals.
Aside from not wanting inexperienced and maybe immature people to take the wheel, adults don't usually want to get into business with minors. That'd make them too vulnerable. No one wants in with someone who doesn't know what they're doing. Which leads me back to suggesting that you get together with friends for now instead of dealing with things way over your head.