If you are taking on knits for the first time ever you really only need 3 things: a sewing machine, ball point needles, and patience. However there are a lot of fun tools you can get that can make sewing with knits more enjoyable and easy.
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Sewing needles: Using the right needle makes all the difference in sewing. Ball point needles are best for sewing with knits.
Euro-Notions Ball Point Jersey Machine Needles
Sewing feet: Many sewing machines have the option of using different sewing feet for different sewing.
Stretch Stitch Foot (Distinctive Overlock Overcast Sewing Machine Presser Foot )
Cutter foot, works like a serger (Side Cutter II Sewing & Cutting Attachment for Low Shank Sewing Machines)
Serger/coverstitch machines: Sergers sew an overlock stitch that allow for stretch and can be a great tool when sewing with knits as they eliminate the need to sew with a sewing machine. You can also use a coverstitch machine to finish hems and necklines, as they sew a straight line that stretches. You can get combo machines (one machine that does both) you just have to switch the machine in between.
Cover stitch machines: