Assuming that you cannot find this same shirt in a smaller size, you could take in the shirt a few inches along the side seams. A simple way to do this would be to find a male friend who is close to your size.
Then put the shirt on him inside out and with your fingers pinch enough fabric along the left and right side seams to get the fit you want. Use straight pins to hold the pinched fabric in place. Then put the shirt on yourself, still inside out and pinned, to check that the general size is good for you. However do not be tempted to pin it further to fit your shape. Since you are crossplaying a male character, you want to maintain the male shape of the shirt.
Once you're satisfied with the adjustments, sew along the line of pinned fabric to create the new left and right side seams. If you can't use a sewing machine, it can be easily done by hand with simple straight stitches.
Finally, try the shirt on again. If everything is good, then you can cut off the excess fabric along the new left and right seams.
The sewing for this is pretty simple, so some basic sewing tutorial videos on YouTube should be enough to get you through that part.
Good luck with your cosplay!