We all had to start at some point. You will learn while doing.
If you are just starting out I would recommend you start with using GIMP, it's a free and open source image manipulation program. It does not have all the fancy stuff from photoshop, but covers all the basics.
You mention wanting to do a NIER cover, I would say that's a good first start. And to make it easier for you, here's a XBOX One template which GIMP can open.
No, I made this. One of the artists at naughty dog drew that picture as fan-art, and as a pitch for the last of us 2 to the director of the team. Edit: Link http://www.gamespot.com/articles/the-last-of-us-fanart-shows-an-older-ellie-naughty-dog-creative-lead-says-i-d-play-it/1100-6419452/
Ah man, a blast from the past for me. I loved that game, and the final drop to earth - so good =)
That said, amazing what you were able to do with Preview. Shameless plug for the GNU Image Manupulation Program, F/LOSS Version of Photoshop. Maybe you want to give it a try =)
well, for $35 (limited time offer now that I just checked their website.) I use Affinity Photo/Designer for all my needs.
Closely comparable to photoshop/illustrator
If you're printing on an inkjet printer, you need inkjet paper to get the best results. If you print on something like this, you'll get much better colors to the point that often times you can't tell you did it at home. Paper, not machine, makes a lot of the difference I've found.