Tricks for Sewing Knits
It helps if you use a straight stitch presser foot on the machine, and also a straight stitch throat plate, if you have one. The straight stitch presser foot has a narrow channel, not the wide slot used when zigzag stitching. The straight stitch throat plate has only a small hole, not a wide slot that is used when doing a zigzag stitch.
It will also help if you use a fine, ball pointed sewing machine needle.
Don't start sewing right at the edge of the material. Instead back stitch a few stitches to the edge and then go forward. Also be sure to keep both the thread from the needle and the thread from the bottom pulled to the back as you start to sew, to prevent snarling of the thread.