Selling printed shirts
yes it is a good way of earning money
example you buy the blank shirt (for $10) 
then you print your design on it ($10)
these numbers are just imaginary ,
ok now total 20 dollar , you can sell the shirt for 40/30 so you have the profit 
you can sell to your friends and family
you can start online 

you can advertise about it in many ways
some are
        facebook
                 facebook is one of the biggest social media , you can advertise your shirt by people like give them little
                 comission each time they buy your shirt
        instagram 
                 if you have budget you can run ads on instagram and target the 13-26 age factor because
                 they are interested in buying new designs
        pinterest  
                 people usually look up pinterest for designs or etc , 
                 you can also write about your shirt on it, make posts about your design.
                 
if you don't know designing you can learn from youtube , there are many photoshops tutorials about it