Can off-the-rack clothes be tailored?

I was watching an "X-treme Makeovers" rerun last night, and I saw a
stylist advising a guy to have his tie-dyed T-shirts tailored. I'd
never considered this possibility, but now that it's in my head...

I wear a lot of T-shirts (very few button-up shirts). I'm willing to
invest the time and effort into learning how to tailor my shirts, but
I have no clue where to search for books or instruction. A cursory
pass on the Internet turned up nothing of value. Can anyone guide me
in the right direction? Is this even /possible?/