If you are truly interested in music creation, I would recommend Ubuntu Studio. It contains pretty much everything one might need for audio work in Ubuntu. While it includes simple programs such as Linux Multimedia Studio, it also supplies the user with JACK, usually necessary for connecting audio and running at lower latencies.
Other key programs include Rosegarden, a MIDI composer, Ardour, Linux's main Digital Audio Workstation, Hydrogen for more composition and beats, etc.
There are also a multitude of synths, most formidably (in my opinion): ZynAddSubFx and Fluidsynth.
If you're feeling especially bold and do not mind paying, energyXT is supposed to be quite simple and great for all of the above. However, certainly for a beginner, I would try the free, opensource programs Ubuntu offers first.
Here are a few much more general guides to linux and audio production:
Here is the main forum where ubuntu audio production questions and answers can be found:
Ubuntu Studio Forum.