There's also the option of going for more of a traditional setup. You can scan it or use a decent camera (think above four megapixels, plenty of lighting) then clean and color correct the image in a program like GIMP (there are at least a couple pertinent guides below). The watercolor in A Redtail's Dream and many of the pages of Holy hand grenade's highly entertaining series Mokepon are traditional art that's been scanned onto a computer.
Otherwise you can consider lightweight options like deviantart's muro in browser drawing app (with a nice autosave function), or look for other browser based solutions.
Here's some general art tutorials (relevant stuff from what I have squirreled away). Take everything with a grain of salt, an artist can only tell you what works best for them. Still, these guys know their stuff:
http://www.questionablecontent.net/tutorial.php here's how the guy from questionable content does his stuff. It's a solid intro on how he does cell shading (take or leave the bpelt plugin).
http://mikeinel.deviantart.com/art/Anime-soft-shading-tutorial-93742741?q=gallery%3Amikeinel%2F2527364&qo=114 this guy's insane. Check out his coloring process. You might not end up using it, but you will learn something.
http://elsevilla.deviantart.com/journal/Get-my-Painters-brushes-231158583 elsevilla has something important to say if you were wondering about digital brush settings in your art program of choice.
http://www.minnasundberg.fi/water_tutorial.htm minna sundberg: teaching you how to draw some waters
http://www.redmoonrising.org/extras/tutorials/ useful tutorials on all parts of creating a webcomic from the author of red moon rising
http://endling.deviantart.com/art/Body-Language-Meme-Envy-301137193 A short note on body language and a great many examples
http://www.livestream.com/Mikeinel mike inel's livestream, lots of filmed art making by one of the brilliant artists behind katawa shoujo (sometimes NSFW)
http://www.fantasio.info/2012/04/android-legacy-look-walkthrough.html a really neat piece for photorealistic painting on top of photo images (sorta NSFW)
http://emptyeasel.com/2007/01/19/how-to-photograph-your-artwork-for-a-portfolio-or-the-internet/ a guide on art photography. Or scanning images with a camera.
http://annadittmann.tumblr.com/post/59321683198/gold-a-color-walk-through-someone-asked-me Guide on reworking a color scheme (photoshop, but applicable anywhere). Try different layers and settings for unification and mood changes.
Vector Stuff
http://mariowibisono.deviantart.com/art/MVP-Tutorial-Chapter-1-47016582 short intro to vector shading
http://photoshopcafe.com/tutorials/pen/pen.htm photoshop pen tutorial. Learn to use it. This is a very powerful tool. (this is "vectors" in photoshop).
http://www.tutorial9.net/tutorials/photoshop-tutorials/pen-tool-basics-in-photoshop/ another photoshop pen tool guide. seriously this thing is useful. (look up pen tool specific to your program of choice)
http://inkscapetutorials.org/ inkscape is an open source adobe illustrator (vector art) alternative. Can't say i've had all that much success with it, but i tried it without training then picked up a copy of illustrator for classwork so I might not have given it a fair shake.
Somewhat Photoshop specific (applies to other programs, just often under different labels)
http://www.lunacore.com/photoshop/tutorials/tut003.htm layer masks in photoshop. very useful.
http://www.cambridgeincolour.com/tutorials/photoshop-curves.htm color correcting tools in photoshop, also very useful