Knit fabrics can be a very forgiving fabric to sew with. The degree of stretch (how much the fabric can be pulled out and recover from) make fitting a garment to your individual shape a bit easier. I have a knit twill skirt that has box pleats and low recovery, it was a bad choice but the print was so much fun I went against my better judgment. When I wear this skirt, at the end of the day it’s pleats are a bit saggy and the garment is heavy. However it has vintage Bakelite buttons and pockets so in the closet rotation it stays. Stretch and recovery vary from fabric to fabric but the overall structure of knit (the loops and chains instead of the x and y-axis of woven fabrics) help reduce fraying and makes the wearing ease flexible and a useful material to master.
The patterns that call for knit fabric usually have negative ease calculated into the pattern measurements unlike with woven you have an average ease of an added 3-4 inches for your measurements. Sometimes when you cut out your size (that you have measured your body for, don’t use your store size, please) the paper may seem a bit small, that is negative ease. Your chosen fabric will stretch on your form, unlike woven fabric which must be fit to your form with seams, darts and design lines.
Knits also come in many different substrates (the base fabric that is printed on, dyed or knit) and some of the names may be regional. Tee shirts are often sewn in an interlock, jersey or tissue weight jersey. Some of the more stable knits that are stretchy but not as much as interlock are ponte de roma, sweatshirt knit, sweatshirt terry, fleece, and scuba knit. Then there are the knits we use in lingerie and athletic sewing like power mesh, tulle Leonardo, swimwear lining, swimwear lycra, dance lycra (which might have more foils and shiny treatments than swim), nude mesh and some nylon tricots (pronounced tree coat, tricot has a bit of mechanical stretch but not much recovery).
There are some wovens that behave like a medium weight knit because they have added into the weave fibers like lycra, spandex or polyester. Some of these would be called stretch denim, stretch twill, stretch shirting, and stretch velvet. These substrates do have some fraying tendencies so treating your seam allowance is necessary.
When sewing knit fabrics on your machine there are needles you need to use called ballpoint or stretch needles. They have different numbered sizes for light to heavy weight fabrics. They are a necessity because the tip of the needle is rounded and will push though the loops and chains instead of cutting them. If you forget to use the right needle you will notice your seams begin to wear out and tear at the stitched thread line.
Machine wise you don’t have to have a serger, a straight stitch lengthened or a zigzag stitch will work well. Many of my projects are made on my sewing machine with a zigzag. One tool for your machine that is very helpful with sewing knits is the accessory foot called a walking foot. When your fabric is fed through the machine it is lightly gripped by feed dogs, those grippy teeth like things below the foot. The walking foot has extra feed dogs on the foot so that both the top and bottom fabric are fed equally. Another way to reduce the drag from the presser foot is to reduce it’s downward pressure. Each machine is different and may or may not allow this, look in your manual or online for tips.
Knit clothing items are some of the most forgiving of all the sewing garments, I think because the fiber stretches giving you up to an inch or more of wiggle room. Understanding the proper tools and tips will get you started with this versatile substrate. The forgiving fibers in knits stretch around your form, fitting you and giving room to breath. Some give and recover their shape very quickly and easily, these usually have more spandex and lycra content (the end of the bolt of fabric will sometimes tell you the percentage). When you make a pair of underwear for instance you want enough spandex or lycra that while you wear them throughout the day they don’t sag down your bum (trust me I have made this mistake).