The best way to spend your free time is doing what you love and working towards something.
Think about the things you have fun with and enjoy doing. Also there must be some sort of ambition or dream you have. Why not figure out what it's going to take to get there and then work on it.
Of course, if what you are really asking is how to be more productive and manage time more efficiently, that is a different question altogether.
I'm struggling with time management myself, but the basic idea for managing time is to journal what you spend your time on every single day. Log the things that you're doing from the time you wake up till the time you go to sleep. That way you can track how much of your time you are spending on any activity. Then you can visually see exactly where you waste the most time doing unproductive shit. Then work on cutting that out and spend the time actually doing something productive.