It depends what you want to develop
If its applications like the ones that make up tinycorelinux then c, c++, and bash are an absolute must.
If you want cross operate system compatibility java can work also.
I beleive you can google c++ tutorials or bash tutorials and find some pretty decent ones. Also you can search for books on amazon.
If anyone has any pointers on how to get started I'd love to know.
I'm a complete noob when it comes to programming so I'm sure others can give you better/more useful advice.