Look on the advanced Techniques page for a video to use knit in the round with circulars, you can also knit a small tube with them too, that's the Magic Loop video.
But it's very easy, when you've done the cast on sts, take the end with the working yarn (that goes back to the skein of yarn) in your right hand, and the other end in your left. You need enough sts to fit around the needle, or else look at Magic loop. Make sure the sts aren't twisted around the needle, and knit into the st on the left needle. Then the next, and keep going.
Oh, and when you knit in the round, when you knit all rounds, you get stockinette st, to get garter stitch, you would knit one round then purl one round.
sue- knitting heretic