How do you decide what kind of material of yarn and/or knitting needles to use in your project? 
What size knitting needles and yarn? How do you see which gauge and size (if not the same thing) if it didn’t have any packaging?
Most of the time the yarn wrapper will have a suggested needle size. Sometimes I’ll follow those recommendations, sometimes not. Depends on what I’m making. Usually with regular worsted weight (it’ll be marked “4” on the wrapper) a size 8 is a general recommendation. The thicker the yarn the bigger the needle. Sometimes though I want something really losely knitted and I’ll just use a bigger needle and smaller yarn. It’s all up to you and what you want your item to look like when you’re done! :knitting:
If the yarn has no wrapper you’ll have to make an educated guess about what it is. Compare it to other yarns you have, too. You can also knit a swatch and see how the fabric feels with that size needle and then change if it’s too loose or too dense.
If it has a wrapper then the information to get you started is there.
Here are some helpful links.
http://www.craftyarncouncil.com/label.html
http://www.craftyarncouncil.com/weight.html
http://www.skeinlane.com/knittingresources/care/caresymbols.htm
http://www.sewingitall.com/knitting-yarn.html
Hi there!-- sounds like you’re a beginning knitter, so here are some tips.
Choosing a yarn has to do with many things. If you can’t wear wool, or don’t like the way some fibers feel, then you won’t want to knit for yourself with them. A pattern will suggest a yarn. You don’t have to use it! If it isn’t clear from the pattern how thick the yarn is supposed to be, then you can look it up on www.yarndex.com and that will tell you. Then you can match it up to what you really do want to use. Let’s say it’s a sort of bulky yarn (5). You can choose another 5 type yarn. Just be aware that those numbers are pretty general, and so if you choose a different yarn, it won’t come out exactly like the one you’re supposed to use. There is also something called “drape”, which is how the yarn feels once it’s knit up into fabric. You could use the same weights of yarns and the same needle size and end up with a stiffer or softer fabric, depending on what the yarn was made of and how it was spun. With experience, you’ll get a sense of what will work.
If the yarn doesn’t have a label, you can use this wpi chart (wraps per inch) at the bottom of this page, to determine whether it’s a thin, medium or regular yarn.