Can anyone give me some adivce on making a lot of DIY t-shirts?

Answer 1

There are tons of books out there now for people such as yourself looking for cute ways to alter clothes.
One that I like is called "99 ways to cut, sew, trim, and tie your shirt into something special"
After a while, you'll get the hang of things and start coming up with your own ideas and designs.
Have fun!