I'll second the recommendation to get on Ravelry. There are loads of free patterns and pictures of things people have made, comments, etc. that will help you decide what to do. Another good resource is craftsy.com, and believe it or not, youtube has loads of knitting how-to videos if you're trying a new technique.
As for how much yarn you'll need, that depends largely on what the thickness of the yarn is and what stitch you're using. Something lacy (which doesn't need to be complex or scary - there are simple lace patterns you can make with bulky yarns that knit up quickly) will require less yardage than something dense and chunky, such as seed stitch.
Given that you've not knit a throw before, I'd suggest finding a pattern you like and working from there. That way you get some guidance as to what needles to use, how much yarn you'll need, etc. In addition to the resources I mentioned above, Vogue knitting has free patterns online, including some old ones from their Knit Simple magazine. It may also be worth looking in knitting books and back issues of knitting magazines at your local library. You don't have to use the exact yarns called for in the pattern - I've had good success substituting more economical ones that have similar characteristics and knit up to the same gauge as the ones called for in published patterns.
Some friendly advice about swatching - if all you've done so far are smaller projects, you may not have needed to worry much about doing gauge swatches. For a larger project, it can be really helpful, because it's no fun having to go out and try to track down yarn from the same dye lot because you've run short. (You probably don't need to ask how I know this...) If your gauge is too small or too large, you can adjust by changing needle sizes up or down. Personally, I have learned that I usually need to go up a needle size to make gauge, unless I'm cabling. Then I need to go up 2 sizes. The only way to learn what your personal gauge adjustments need to be is to play a little with the yarn.
As for what yarn to use, I'd steer away from most 100% acrylics because they tend to pill and really aren't that warm. If I'm looking for economy and easy care, I tend to go with washable acrylic/wool blends. Then you get the warmth, breathability, and hand of wool with the easy care and price of acrylic. If you decide to use cotton, double check and make sure it's an easy care cotton. Some cottons are hand wash.
I hope that helps. Feel free to PM me with questions if you'd like.
