After selecting a project you want to knit yourself, examine the garment very well so you could choose proper yarn for it (that is why I said that selecting real clothing piece for the first knitting project is the best for the beginners). Take it with you when buying yarn (please, no online shops for the first time..), so you could compare few yarns and have a better understanding of what you need.
Thickness. Thicker the yarn – thicker the knit. It is partly true – actually the thickness of your knit depends on yarn and needle combination. Select your needles considering what kind of a knitter you are – if you like tight knitting style, you probably should take needles which are thicker than your yarn. And if you are loose knitter – thinner that your yarn. For example, I am tight knitting style lover, so I usually choose thicker needles for soft, gentle and smooth garment, lets say, a scarf, and same thickness as my yarn or more structured result.
When talking about mohair – needles are usually two three times thicker, when knitting a shawl, 1,5 times thicker when knitting a sweater.
Also, if you have different thickness needles, you may knit samples with all of them and decide then which pair to choose. I would advise not to use thicker needles for more structured and tight fit clothing pieces because big loops is not what you want to achieve here and the finished result will look kind of weird.
very tight – medium- loose
Spinning method. Seems strange but this is a detail to pay your attention to. Harder yarn can help to emphasise texture, if this is what you are going for. Choosing thinner needles could double the effect. Selecting looser yarn would create softer material. There are also slightly felted yarns (look like loose fiber) – thicker would perfectly fit for bigger projects that you could felt after you are done, and thinner would nicely work in shawls. Do not use this kind of yarn for sweaters or other clothing pieces, because unbelted garment is really weak and can easily be damaged.
Materials. For the first time I suggest to choose wool os thick cotton. Natural materials are nicer to touch and the yarn itself is less slippery comparing with synthetical yarn. When talking about needles – non-slippery surface is ok, if you are not afraid your loops to slip way, but it will prevent you from faster knitting. So I would suggest you to try square metallic needles. For those who want lighter tools (which I do not prefer), bamboo needles will do.
Hope these two tips helps you to choose your yarn and tools!