Look carefully at the various ways of doing this. You don't neccessarily have to get a bunch of t-shirts printed in various sizes up front and them sit staring at them for years as you try to sell them.
There are companies that make merchandising ("merch") for amateurs and small "operators", youtubers in particular, where you upload your design to their site, link wannabe buyers to it, and they then print out the t-shirts or whatever TO ORDER, in return for their cut, of course.
The advantages are:
you don't have to run a webshop/shipping operation yourself
no minimum order level
no up-front investment
no risk of over-stocking
greater variety of merch (t-shirts, mugs, wristbands, whatever).
Issue to consider are:
how much do they charge for purchase and shipping
how much percentage do they want
where do they ship from: you don't want postage costs to make it unattractive from the majority of customers. Some companies will produce some items locally to the customers, i.e. if it's a US company with a UK buyer, they'll sub-contract the job to a UK printer who'll then only charge local postage, rather than having to pay shipping and tax from the USA.
Note that I've no personal experience of using these services and am not recommending any particular one:
This is a company that a lot of youtubers use:
https://www.districtlines.comOther companies do it on a crowdfunding basis, similar to Kickstarter. This is one example:
https://represent.com/
