Mysterious extra stitches are very common among new knitters, so common in fact that we have a sticky thread devoted to it so check that out.
What often happens is you somehow don't notice that the yarn is in front or between the needles and when you knit with it like that it produces what is called a yarn over (YO). A YO is an increase stitch. This may not be your problem, but it's something to watch for. Keep the yarn in the back when knitting. It only comes forward between the needles for certain stitches like purling or YO.