A sock needs a VERY STRETCHY cast-on (unless you want a sock that cuts off your circulation). You can try to do a long-tail cast-on loosely enough that it will work, or you can do other tricks, like casting on 50% more stitches than you need and then decreasing them away in the first round. OR – you can just do a cast-on that is inherently more stretchy.
My favorite stretchy cast on for socks (and hats and gloves and anything else that needs a LOT of stretch) is the Twisted German cast-on, also sometimes called the Twisted Norwegian cast-on, or as my friend and former boss Melanie called it, the Twisted Drunken Crazy German cast-on.
So, without further adieu, here ’tis! The second video shows how to join stitches to knit in the round and conceal the jog at the beginning of the round. ENJOY!