.. once u get used to it, it's straightforward but some of the terminolgy takes practice. Audacity is like an old multi track recorder with unlimited channels and a massive level of editing. One you've got it open the help function (top right) is really really good.
You can import mp3 files or wav or whatever. When you save an open activity in Audacity, it saves it as a Audacity project file so that when u open it next time u can continue to edit etc. If u want to create mp3 or wav or whatever just use the export function not just save.
As an aside, I'd normally use FL Studio and or Studio One for creating music but because i use Linux rather than Windows, Audacity can be used in both I'm discovering how good Audacity is. Linux for music has previously been impenetrable for me so this is all good stuff.
I've got a Focusrite sound card and Audacity always works and easily. Just plug a mic or guitar in and press record.
One thing I use Audacity for is transferring my old saggy cassette based songs to PC. Then I can add reverb, change tempo, repair the mix, repair holes add compression etc etc etc etc ...
Good luck !