I know I'm a bit late to the party but Evil is using illustration/graphic and UI design in the creation of these pieces, so look up tutorials and examples of both.
Easy/Short Answer:
1. Get Photoshop/GIMP and practice using tutorials. I used sites like Pixel2Life and PSTuts 10 years ago and they haven't changed much now.
2. Get some pen and paper and draw. Draw anything but try to keep it real. Fantasy is just exaggerated reality afterall.
Hard/Long Answer:
Design and art in general is what you make of it. When you get into all the nooks and crannies design can be really complicated stuff but so is everything, and overall all we're really doing is drawing. That's it. Either we're using pen and paper to draw or a graphics tablet, so just get one and draw!
When I was learning I just used Pixel2Life.com and read tutorials on shit that interested me, which at the time was a lot of the cheesy forum signatures and banners (we're talking a good 10 years ago now). Then I went and looked at a bunch of free designs (and deviantART accounts like this) and tried making my own. Take into account the -techniques- you're learning from these and not just the outcomes themselves as these techniques are what you'll use, rather than the end product itself.
For basic drawing you'll want to sketch at least once a day, everyday, for 5 days a week. You DO NOT have to do it that much, you just want to do it REGULARLY and OFTEN. You don't need to learn traditional pen and paper sketches before you get a tablet (as you'll have to get used to the tablet pen anyway) but it's good practice anyway. I do all my drafts in pen and pencil, scan them to a PC and then use the tablet then. If you have a tablet consider taking other peoples work and try inking and colouring it, as you'll want to learn that, too (remember, colouring/shading is a real job, too!)
In regards to what to draw I suggest drawing -real- things. Anything, doesn't matter, so long as it's real. Photographs work too but usually have unnatural lightning as opposed to something that's in your house. Pick a weekly topic (like "fruit" or "kitchen appliances") and just draw that for the whole week. If you want to draw people and animals don't start with the whole thing, start with a body part like hands, feet, horns, teeth, eyes, etc.
Like any new hobby, you need to be sure you want to do this. Learning a new thing is quite involved and hard, so just be patient and take it slow. When I started it took me about two years to get comfortable with Photoshop, and even then it took me about 3-4 before I was comfortable making my own layouts. Nowadays I do more logo/poster work than UI stuff, which is really strange since I absolutely suck at drawing!
Extra reading: Design elements and principles, especially in regards to colour theory and basic UI design. Colour theory's a good one. Also look up mindmapping and ideation if you're stuck on what to create or how to go about it.