Yes. I shop for clothes online all the time! The key is to know if you fit into the clothing by trying other things from the same brand and checking the size. for instance, if i had a shirt from gap that was an xs, and i wanted to get a new shirt, i'd get an xs. I usually buy my clothes online at old navy, and they always work because almost every time they fit perfectly, but if they don't you can get a full refund or an exchange with free shipping! : )
I would start with the simple designs, stencil 1-2 colours and screen print. By the time you get screen, ink, squeegees, an iron and cleaning agents you have invested maybe about $50 in equipment. You can buy blank shirts for a couple bucks each.
There will be a bit of trail and error until you are able to get an even print, however once you get it down you will be able to mass produce for not a lot of money.
Many places still use screen printing for doing t-shirt design, it is still by far the cheapest most efficient way to go about it.
It is also a good DIY project. Outsourcing small scale production is extremely expensive.
As a side note, you can reuse the same screen many many times. So it is a one time purchase which will allow you to do several different simple designs.