There are many ways to hold your needles, experiment to see what feels comfortable to you. The important thing is that stitches are moved from the needle in the left hand over to the needle in the right hand every time a new stitch is made.

Transcript:

It is easy to accidentally knit two stitches into the first stitch of a new row by taking the yarn back over the top of the left-hand needle. This pulls the two legs of the first stitch on the previous row up onto the needle making it look like there are two stitches.

To avoid this, pull the yarn down and to the back of your needle before inserting the right-hand needle into the first stitch to start a row.