****For the side seam, use a stitch that has at least some forward/backward-motion, it puts more thread into the seam and makes it elastic. These stitches should work fine: mode 1 #5 or #6 mode 3 #1, #2, #3 the stitches in mode 3 finish the edges.
The hem: press the hem, then sew with an elastic stitch or the twin needle. Personally, I don't like the twin needle, I rather use some nice decorative stitches like mode 2 #27 #29 #31 #32 #46 #49 #59 #67 mode 3 #28 #45 or any other stitch you like.
Remember to loosen the presser foot pressure for sewing knits to prevent stretching and to use jersey or stretch needles.****
I have made a few mistakes but I have now done 10 t shirts and I'm happily wearing them. The only thing I have ever sewn is a few quilts, wall hangings, curtains once, and purses. I have never done anything with clothing so this was a first. Susie in northern NY