You want to get a headstart? That is an awesome idea! Pick something that works for you -- be dedicated, but if one thing is not piquing your interest, come at it from another angle.
Lots of people have heard of Duolingo. Try it out, some kids love it. On an app or on the website. https://www.duolingo.com/
I prefer Memrise. It's as an app or on the website. A lot of offerings are free. I'd recommend trying out one or more of these courses: https://www.memrise.com/courses/english/german-2/ (Why Memrise? I am trying to learn Turkish, and I seem to be more comfortable with the Memrise style, learning words and short phrases -- and to keep it exciting, I can do multiple sections/units at a time to keep my brain interested, and to learn vocab I want to learn.