I'm an incurable cross-stitcher
I've been stitching since I was 14, and have always been drawn to what I call Epic Projects: big designs on high-count fabrics. (Because I'm a glutton for punishment, that's why!)
Anyway. The basics of cross stitch are easy to learn, and most patterns come with instructions for the basic stitches. An afghan has the advantage of larger "holes" and lower thread count, so each section will go more quickly than if you were doing the project on, say, silk gauze.
You will be doing A LOT of "sections" on an afghan, though, so expect to invest some time into this project. I don't think it's too ambitious, but be aware that the size, alone, will have you stitching for several months to come.
I really like About's cross stitch site. There's a library of stitches and lots of wonderful information there, including "reversible" cross stitch (a method I've never bothered with on afghans, but I hear makes the back indistinguishable from the front).
My tips to you (besides telling you to check out the About site) are these: be patient with yourself. Be willing to rip out your work, if you have to. (It's OK. We ALL do it. It's called Frogging - rip-it, rip-it.) And don't carry your threads across the back of the work for more than a few squares on your canvas. (I pride myself on keeping the back of my work neat; my mom, who is a much more advanced stitcher, worries about it much less. It's largely a matter of personal preference -- the Cross Stitch Police are not going to come after you!)
Most of all, enjoy your new hobby.