Have you been knitting in the round up until this point?
If the instructions say something like "knit all rounds", then what you're doing is switching from knitting in ribbing to knitting in stockinette st (the outside of the sock will all be "V"s, and the inside will all be purl bumps).
Do the instructions come with a photo? If the socks look like this in the picture http://www.knitlist.com/97gift/menssocks.htm , then switch to knitting all the sts (instead of the K1P1 you've been doing). But if the picture looks like this: http://sarah-knits.blogspot.com/2006/06/toe-up-sock-pattern-invisible-double.html , then you need to keep doing the k1p1.