How are you doing Maya, have you tried sewing it? You could also try hem tape, which is done with ironing.
If you want to try sewing, you'll need some white or light thread and a sewing needle. You might find it easier to work with a fairly large needle, they tend to have large thread holes. Unwind about 2-3' of thread (much more will tangle). Cut the thread off the spool. When threading the needle it helps to use the newly cut end.
Now thread the needle. Try closing one eye and holding the needle up to a light if you have trouble. Hold your finger close to the end of the thread with just a bit sticking out and put it through the hole. Once you get it through pull about 6-8" through the needle. No need to knot it, just keep your eye on it while you're sewing so the thread doesn't fall out of the needle.
Now, if you've marked your shoulder seams with a pin you're ready to start sewing.
Keep the pin in while you sew, just watch your fingers. Remember to have the excess fabric (seam) inside the shirt. Now flip the shirt inside out to begin sewing.
Start on the inside end of the seam (which would be towards your neck) and have the front of the shirt facing you. Insert the needle through both pieces of fabric, away from you, close to the edge. Pull nearly all the way through but leave a tail of thread hanging (about 3-4").
Now bring the needle back toward you and come back through both pieces of fabric a small distance away from the first entry point (smaller is generally better, but 1/4-1/2" will be fine to start). Keep doing this working your way slowly down the seam. It may be easier for you if you flip the fabric for each stitch so you can see what you're doing.
Since this is a shoulder seam, when you get to the end throw an extra stitch or two on the end to stabilize it and then work your way back down the fabric. Make your last stitch so that the thread matches up to the tail you left when you started. Cut the thread from your needle, leaving a few inches. Tie those two tails together (knot firmly but don't pull the thread too much or if will bunch up the fabric). Trim the excess thread but leave a bit (1/2") so it doesn't unravel.
When you're happy with the result you can trim the fabric from inside the seam, but I would take your time with this step. Knit stretchy fabric and shoulder seams aren't the easiest to sew so if you want to practice seams, you might try a slightly stiff cotton or something like that. Hemming pants or skirts is actually fairly easy once you get the hang of it.
I hope that helps! Good luck!
