The needle size on the yarn ball is a recommendation for that yarn weight. You may knit more tightly or more loosely and so need to change the needle size in order to get the gauge or stitches per inch that you want or that a pattern calls for.
Yes, those pesky extra stitches
. See if you can tell where the extra stitch occurs along the current or previous rows. Sometimes it's a yarn over that happens when you change from knits to purls or vice versa. Instead of bringing the yarn between
the two needles as you should, the yarn is brought over the right needle and that creates an extra stitch. The other common place for extra sts is at the beginning of a row. If you pull the yarn up from the left needle, the single stitch at the needle tip looks like two sts. Knitting into both legs will give you an increase. Pull the yarn down and to the side or back so that you can see that there's only one stitch there.
Using markers can help with counting sts (and locating extra sts). These can be as simple as a loop of yarn with a knot. Put them on the needle every 10 or 20sts to make counting easier. Counting sts is a very good practice for checking your work and finding mistakes early on.
You're learning a lot with each row. Good luck with it!