Linux is a whole different OS to Windows and OSX. You can download Ubuntu Studio (or any other Linux OS) for free and you probably should install the OS on a separate partion on your hard drive, that way you can keep your current OS.
So now you need some apps. The good news is that there are those that will easily replace your old Windows/OSX programs, and there are some which are also on Windows and OSX as well. For example if you use Audacity or Renoise (€49) then you are in luck because you can get Linux versions of those.
Then if you like Pro Tools you might be interested in Ardour, which some say is better than Pro Tools.
energyXT2 is probably one of the most impressive looking DAWs around, unfortunately it costs money (€19-49 depending on the version you choose) but it's still cheap, and it runs on Windows and OSX as well which is handy if you ever want to switch back.
There is ZynAddSubFX, which is an impressive looking synth.
For drum machine needs there is Hydrogen.
And there is also Rosegarden for MIDI work.
Of course there are heaps more (there's a list here) but these seem to be among the best. Unfortunately there does seem to be a bit of a problem with running VST's on Linux thatnks to Steinberg which I don't fully understand how to fix.
But there is a bit of information about using VST's here(you can also find out VST compatibility info here). Unfortunately in my search for more information that made some form of sense, I mostly found outdated info, though there is this informative article.