There are so many variables (hook size, gauge, type of yarn, pattern itself) that it's hard to say. When I don't know how much yarn I need, I buy one skein and see how far it takes me. Then I buy more based on my calculations.
Like "2dc in same st" or something like that? Don't overthink it. It's two double stitches in the same hole. Make the first stitch like you normally would, then make the second stitch in the same hole that you used for the first stitch. Make sense?
My grandma taught me to chain when I was about 8, I think. I made dozens of "bookmarks" and then promptly forgot it.
When I was in college, my roommate and I spent a summer practically crawling up the walls in a new apartment because we didn't have television or internet installed. In an act of desperation my roommate taught me how to single stitch, just to do something to pass the time. I made a horrible-looking scarf with uneven stitches and ragged edges. Then I bought a crochet book, taught myself to double stitch, and made a truly hideous afghan throw blanket. From there I've taught myself new stitches and techniques with the help of patterns and online tutorials.