AHEM, a screenshot of a real game is just a screenshot. A mockup is a picture drawn to LOOK like a real game, but isn't.
Programs: Matter of preference. Though, the big four programs mostly used, here atleast, are Photoshop, GraphicsGale, ProMotion, and MSPaint. I do not recommend MSPaint.
Photoshop isn't really intended for pixel art, but it's certainly got its merits when it comes to the matter.
GraphicsGale is what I use. Great pallete control, customizable short-cut keys. Pretty basic, but it's got everything you need.
ProMotion is some big-time stuff. It's shareware. Pro-version costs a ton (Well, money is relative), but it's got tons of awesome features unique to it. There's also a lighter version, which has pretty much the same functionality as GraphicsGale.
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Forum is borked at the moment, so you can't choose an avatar.
As for where you should start, just draw some pixel art and post it here (You could also check out the line-art thread if strapped for ideas). So long as you'll listen, you can learn all the basics pretty orangatang'd fast.
Andddd, don't take criticism on an emotional level. Critics are made to help you, not harm you, and when you don't listen to 'em, you're harming your growth as an artist.
A'course, you don't HAVE to listen to 'em, and bad advice has been given before, but the gist of what I'm trying to say is don't limit yourself behind excuses. (I can never properly convey what I mean when typing, gorrammit.)
