+1 for musictheory.net. That's an amazing resource for theory. My suggestion would be to learn what you can about chord progression and then start dissecting your favorite songs. A simple way to play piano is melody on the right hand and chords on the left. Once you figure out how to tell the chord progression of a song you can play almost anything! Another thing you can do is to a site like
chordie and look up some songs there. They'll give you the chords and, again, chords on the left hand melody on the right.
As for software, Garage Band on the mac is a GREAT way to start messing around with things. On the windows side of things, I would suggest
Studio One. To be honest, I've never used it, so I can't speak from experience. But, looking around at free DAW's it seems to offer more than most. If anyone has tried Studio One feel free to comment, I'm curious.
Best of luck with your musical endeavors!