craftgrrl | knitting help: switching from knit to purl and back

I have tried over and over again to learn how to knit and it never seems to work. I am trying yet again hehe. I got much larger gauge needles this timeand it seems toreally make things easier. I can knit for severalrows just fine adn then I can also purl for severalrows at a time (which I knwo is essentially the same thing).
So now I just want to make a basic ribbed scarf: ya know knit 2 purl 2 etc. The problem is I can't for the life of me figure out what I am supposed to do with the yarn when I swutch from purl to knit. I seem to either manage to wrap it around the needle in such a way that it looks like I have an extra stitch, or I end up tying up what I have already knitted. I looked thorugh the memories and went to a bunch ofthe knitting sites that are suggested. There is a lot of good info on them, but nothing showing the transition between purling and knitting. I feel like I am thinking this to death. I'm sure the answer is really obvious, but I can't for the life of me figure it out...
Also, how do you all cast on for a ribbed scarf? I just knitted on, but that seems to hinder the stretch of the rib knit so I'm wondering if there is a better way, or if it will work itself out once I have knitted more (assuming I can figure out how).