Welcome to KH!
There are many ways of picking up extra stitches. Two of the most common are extra sts at the end of row and sccidntal yarn overs.
Make sure that when you begin a row, you pull the working yarn down so that you see the stitch at the end of the needle and not up, which makes that stitch look like two sts.
If you're changing between knit and purl sts, be sure to bring the yarn to the front to purl or to the back to knit, between the needles and not over them, creating a yarn over.
Keep count of your sts after each row so that you can try to identify where the extra sts are coming from.
Here's a link to some more possible causes.