how should i use my knowledge of a computer langague

To be honest, you should make whatever you enjoy. I don't think there's a point in programming for fun unless you're actually enjoying it. Make a text based game if you enjoy it, make programs depending on how good you are with the language. The only real way to improve is by coding and experiencing things. There are also lots of websites with programming problems if you'd like to test your luck with these.

If you want to make minecraft plugins then learn the bukkit API, it's really not that complicated and can be pretty fun.