[mak4athp]

Whatever strikes your fancy. If you're pursuing startup culture at your age, and you aren't trying to become a developer/engineer yourself -- you have all the freedom in the world.

You've probably got more honed skills to contribute actively, and you won't have opportunity to make especially useful contributions programming in anything that's widespread. So you can dable in something popular to be able to read it better, explore something esoteric to have something novel in your toolkit, or just pursue something that's personally practical like the systems scripting and web stuff you've already been toying with.

Don't overthink it. Just find the one that gets you excited and spend whatever time on it you'd like.