They usually use silk screening, single color printing can be done fairly simply.
Below is a hobby kit for $150
Amazon has something called "Speedball Super Value Opaque Fabric Screen Printing Kit" for $30
might be missing the screen making stuff.
Services like cafe press print custom shirts for about $24 each
Ok well you're 17 and wearing girl's clothes. . .i mean i dont judge it's ok but don't you have your own clothes? Just think: what would people in your school say if you're wearing a bra and girl clothes to high school/ college? I won't recommend doing this but if you really want to then i guess it's ok as long as you dont mind people making fun of you :( .
Hope this helped!