Have you chosen your pattern yet?
If you join Ravelry (it's free), you can look at the patterns on there (literally thousands of them) and see which yarns people have used to knit them, and what they look like knitted up (and on people of different ages, shapes and sizes, which I find really useful). They have a lot of free patterns, and even more paid-for ones that you can download.
You can pick a yarn you like when you've seen what it looks like on the pattern you've chosen (and many have links to shops or sites where you can buy it) and then check out the comments from people who have used it.
You can also join one (or more) of the hundreds of forums on there, and ask questions about the yarn. Ravellers come from all over the world, and there is almost definitely going to be someone who will have used it.