Hi Kitty and welcome. Look at the top of this forum and the sticky 'Mysterious Extra Stitches'. The posts in it mention the various ways you can gain sts and how to avoid doing that. I think what you're asking about is to make sure you let the st drop off the left needle after you've knit into it. If you slip both the new and the old st to the right needle, you get an extra one.
sue- knitting heretic