Help a newbie out...unwanted added stitches

[wilspika]

Hi
I have just started knitting recently and am mostly teaching myself. I;ve got the knit and purl stitches down. I can do the stockingette stitch and cast on and all. But I seem to keep adding on stitches inadvertantly. I'll cast on 24 stitches, stay at 24 for a while then for some reason I keep picking up stitches and before I know it I have 30 or 32. And they always seem to add on to the same side and only one side...the other side looks fine.

What am I doing wrong? How can I figure this out?