While knit garments are quite simple in their structure, often requiring no darts or zipper closures, they can be quite tricky to work with especially as a sewing beginner. Perhaps one of the greatest challenges is stitching knit seams that do not break or create tension in the final garment.
When sewing with knit fabrics, it is best to use a zigzag stitch to sew flexible, stretch seams that do not break with extended wear and care. If you have a top of the line, newer sewing machine, you may have access to specialized stitches designed for stretch fabrics like knits. Regardless of which setting you use, the stcrture of teh sticth will be fairly similar allowing enough flexibility in the seam when the garment is stretched.
If you take a closer look at the structure of a zigzag stitch, you will notice that the up nd down chevron pattern provides enough movement so that when the seam is stretched, the stitch expands along with the fabric's movement without creating any tension. No tension means less chance of breakage and thus a longer lasting garment.