I am worried that i will miss my connecting flight.... any advice?

Don't worry, at 13 you should be traveling as an escorted minor. That means a gate agent for United will be your personal guide to make sure you get from one plane to the other. 50 minutes is very common for a change of planes. The airlines take the security and protection of a minor VERY seriously. They will even hold the connecting flight for you, while if an old person is late, they close the door. So, hopefully you won't worry. They will have you well taken care of, and worrying is supposed to be just for parents.