First know that you do not have any limitations than your own exitus ;)
If you have fun writing programs, browse the internet and look for applications which you think of 'man I could improve this' or 'nice tool, but I have some other ideas'.
Then just look after the languages in which these applications are written and do some quick research about the difficulty of creating such an application by yourself (e.g. on stackoverflow.com).
When you have found some project which you really want to set up and you have chosen the language which makes the most sense for writing this application: Start learning this language.
It is not always important to look for a specific language, just look for a project and set it up.
