12 lbs should be enough, but more is always better in my opinion!
there is a calculator on this site in the calculator section that will tell you how much you need.... I'd aim for at least a 3" deep bed.
when you first get it... pour in the sand.... then put a large bowl in it and pour in the water so that it doesnt disturb the sand... and then throw a raw piece of shrimp in for a few days to start the cycle.
6.95 is 4 cents cheaper than at my LFS
My advice would be to get a larger tank if you can because they are much more stable. Other than that just read as much as you can. I'd go and buy the conscientous (sp?) marine aquarist. Great book to start with!
welcome to the site and good luck!!!!!