But as a starter like me who wants to learn and never did anything with linux where do i start?

By the way. i saw some options of these emulators with opengl and stuff.

There must be a good chance that it can be directly ported isn't there? with a same type of CPU in the n900 and also opengl support.

Just tested desmume it runs the newest roms and even has a better video output. so the source should be an excelent foundation for the build of a n900 version