Welcome to Knitting Help!
Few questions first...Are you following a pattern and if so which one? Are you doing garter stitch (knit every row)? Are you a new knitter?
It's common for new knitters to add extra stitches. The most common cause is an increase called a yarn over (YO) which is created by having the working yarn in the front of the work instead of the back. If you are changing between knits and purls make sure the yarn is moved to the back when you're reading to do a knit stitch.