It happens to us all. You find that great T-shirt in the dollar bin that doesn't quite fit. Well no big, here's a beginners solution. Use a shirt that fits you well as a pattern. You will mark on the inside out of the shirt to be taken in (I use soap scraps to mark fabric). Carefully pin the edges. I don't remove the seams at this point. I find that for a beginner, having more structure to the garment while sewing can save some grief.
Sew the new seams and try the shirt on. If it fits, remove the old seams.
I took my shirt in about two inches on each side. The fit was good, but I decided to sew in a small panel of stretch lace so that the scoop neck was a bit more modest. Congratulations! You have a shirt that fits!