The cast on sts will be on the same needle your working yarn is on. Put the needle into the L hand, knit them on - then if you're these are to be at the beginning of the row, purl them, then purl the old sts. If your pattern says to cast on at the end of the row, it's the same thing, just move the needle to the other hand and cast on.
btw, it helps to know what pattern you're knitting, in case the suggestion I gave you isn't right for what you're supposed to do.
sue- knitting heretic