For straight needles, length isn't super important unless you're making an afghan or some other really wide project. My first straight needles were 10 inches or so. That length is perfectly fine for scarves, dish cloths, hats worked flat, etc. Length of needles is more important when you're knitting in the round on circulars; and then it's the length of the cable holding them together that makes up the majority of the length.
For yarn info, I just Googled types of yarn and found several good articles explaining the different weights, textures, and fibers used. For a first project, like a scarf, find a light colored yarn that is smooth (not fuzzy) and worsted weight (4 on the label). A light color will allow you to see the differences in the stitches better and a worsted weight is a nice, medium thickness. It's thick enough for most things like hats, scarves, and sweaters, but not so thick that it's hard to work with.
Check out patterns, the videos here, and ask questions. Have fun with it and enjoy! Pretty soon everything will make sense and you'll love it!
Courage is the magic that turns dreams into reality.