Hi, Kat!
I'm no expert, either, when it comes to garments; however, I have a dress made from Tshirt fabric which is waaayyy too long, so am preparing to shorten and re-hem it. I believe the preferred method is to use a twin needle, for knit (may have a blue band around the shaft). There are two kinds.(2.0 or 4.0, and offhand I can't remember which is which, but...) You want the one with the needles the farthest apart. Now, you will be sewing from the TOP side of the fabric, so make sure to pin really, really well, and I'm going to leave myself at least 1/2" excess, because after you've finished sewing, I think you're supposed to go back and with sharp scissors, trim away the excess close to the stitching line underneath (wrong side of fabric). This will give you the look of a hem on storebought tee shirts, with the two little parallel rows of stitching on the top, and the way the bobbin thread moves between the two needles is going to give you that certain look underneath, like storebought. ...I also think, to thread the machine, you're supposed to use two rolls of thread, but place them so they'll turn opposite directions and thread each needle individually. In other words, don't pull both through the all the thread guides at the same time. This is to prevent their becoming tangled while the machine is sewing.
I hope someone else will chime in here and clarify all this information for you; I'm afraid I may not have made much sense. But please let us know how your project turns out, okay?
Good luck!
twinklenose

Use a twin needle. Any size is ok & they come in several sizes. Sew from the top. Thread as usual except run through the tension disks separtely & through the needles. You will have to have 2 spools of thread & 2 thread holders on top. I don't think all machines accommodate twin needles. I think only the ones threading front to back & not the ones threading to the side.

The stitches form a zigzag underneath which stretches a little without breaking like a single stitch would.

Hi again, Kat!
Thank you, Joan, for explaining about hemming the Tshirts. I hemmed my dress this morning, and it turned out well!:)
I hope you'll give it a try, Kat.:)
Oh, I think I forgot to mention, a professional told me that when using a twin needle, do not backstitch. Is this right, Joan?

Thanks for the help. This sounds a little complicated (for me) but hey, I like a challenge! It will be a week or two until I can get the use of the sewing machine but I'll give it a try. Thanks again & wish me luck! I'll let you know how it goes.

Dear GardenKat,
If you have any decorative stitches on your machine another option is to use a single needle and a decorative stretch stitch. I have one on my machine the looks like you keep writing an e in script over and over, that makes a nice t-shirt hem and you can do it from the back. I always pin and iron before sewing, a little starch helps keep everything where it belongs. Practise on a shirt you don't like, t-shirts make great rags!
Teri

I use the twin needles all the time and have never had a problem with back stitching.
You may find that the ballpoint needles are the only kind that will work on some knits.