Hi!
Rather than tell you what software is "best" to use, (as its down to opinion anyway)
I'll tell you about how I go about making my tracks:
-Audio & MIDI: JACK is an server that does inter application audio / midi routing. It's gonna be the core of any pro-audio system you have on linux. QJackCtl is the frontend to the server, & that's how to launch it all.
-Sequencer software: Qtracktor, handles both audio & midi tracks, has some useful keybindings you'll need to learn before you get the most out of it.
-Audio recording: I usually compose tracks in MIDI, bounce them trough an XV-3080 roland synth, record the audio *back* into Qtractor. That way I can keep that instrument playing, and change the patch on the synth, to record another instrument or 2... Then I export the audio to .wav files, and import them into Ardour. (tip: make the recordings all from the beginning, and align the starts to a beat in Ardour. No more messy aligning the tracks per ear.) In Ardour I make all the automation, effects, mixing volumes etc. Export the track once happy.
-Mastering: Audacity, just load it up, and have a look if your happy with the overall dynamics or if it could benefit from some compression / name it. Export to .ogg, .mp3, .flac, .wav or whatever format you want, type in the track info, done! :)
Hardware wise, the following is *needed* for the above setup:
-Soundcard: can be simple & onboard thing, but for better performance & quality go with a pro-interface. Check compatibility of the card before you splash out!
-MIDI interface: To drive the synth. (Also to record MIDI into QTractor if you so wish)
-Mixer: a hardware mixer isn't strictly needed, but its healthy to have a real fader you can pull down if things go too loud.. :S
-Speakers: but you know that :)
Accessories: Of course you can add on as much hardware as you like, but sometimes its *easier* to do things in software, like automation of parameters on you FX unit over MIDI can be a pain, Ardour takes care of details if you use an LV2 or LADSPA reverb.
Note that there are many many more sequencers, another couple of HDR solutions and hunderds of other workflows available. The best part is that you get to pick a workflow that suits you best. As I've said, I've described my workflow here, no need to stick to it!
Cheers & good luck! -Harry