Welcome to Knitting Help, Andrea!
I always suggest new knitters start with a size US 8 or 9 needle. Choose a smooth, worsted weight (4), light colored yarn to practice with. NO novelty or fuzzy yarns because those are harder to learn on and harder to see your stitches.
As you can see our suggestions overlap a bit so that gives you some leeway when looking for materials.
We have lots of videos here at the tab at the top of the page, plus I have other site suggestions if you find that need more than a video (I did) to learn.
I hadn't thought about the arms on the wheelchair so Suzy's suggestion may be a good idea.