How would you shrink a shirt that's too big? I don't know what it's made of; I've already tried the washer and dryer. Is there any other way?

A seamstress could help...
By taking in from the sides and cutting off a bit from the bottom etc.

Or hot water in the washer and highest heat setting on the dryer may help (cant guarantee it though).Different fabrics may react differently.

I would still recommend the seamstress :)