What do you mean by:
Are you talking about using a circular needle to knit flat, back and forth? You cast on to circular needles like any straight needle, just make yourself believe one end of the circular is a straight needle and do it the way you always do. When you finish a row and are ready to turn, you exchange the two needle ends from one hand to the other. That is the same thing you do with straight needles, but it seems so obvious using the straights but it works the same way with the circular. Just seems a bit weird at first.
If you are trying to knit in the round, ignore all this.