I know a way to do it if you just want to make it tighter:
1. Turn it inside out
(If needed, try it on)
2. Put needles where you want to sew the seam (how tight it will be). Make sure the needles aren't in the same direction as the seam, because otherwise they might stuck in the sewing machine. See pic.
3. If you want/need to, make a line with a pen where the seam is going to be.
4. Sew an overlock seam (zick-zack or just a straight seam might work too, but the fabrics is stretchy and the seam might break when you stretch it) where you've put the needles.
5. Take off the needles.
6. Turn it inside out and enjoy your new skinny fit t-shirt!
Please note that you don't have to sew in the sleeves too, just stop at the armpits.
I do this all the time, because I buy my tees in M (I'm a small girl, lol) because I want the lenght (but I still want them to be a bit tight).