Typically you need to know the stitch gauge of the yarn you're going to use, and the circumference of your ankle to work out how many stitches to cast on.
So... yarn states 30 st per 4 inches. Divide by 4 to get the number of stitches per inch (7.5). Measure round your ankle, not too tightly, let's say it's 8 inches. Multiple the stitch per inch by the ankle circumference.... so 8x7.5 = 60. So you cast on 60 stitches for your sock. Also bear in mind your needles size will make a difference too.
Welcome to the sock addiction!
PS. Also meant to add that these instructions are for top down socks.
Last edited by butlersabroad : 03-18-2013 at 05:36 PM.
Reason: Added more info