It may not seem like a big deal but the weight of your yarn matters ALOT!! You don’t always have to use the recommended needle size but keep in mind that most thinner/lighter yarns require thinner/smaller needles + hooks and chunkier yarns require bigger/fatter needles + hooks.
The Standard Yarn Weight System is a guide that shows you all of the yarn weights from lightest to heaviest. Hope you find this useful!
Below is a PDF file by the Craft Yarn Council
It lists all of the yarn weights from lightest to heaviest. The Craft Yarn Council is a FANTASTIC resource for all things knit and crochet. Be prepared to learn a TON!
Anyway, here ya go:
**photo credit Moogly