You use the ink thats already in your printer
When buying your transfer paper make sure you buy the right type, depending on what kind of printer you have - eg if you have an inkjet printer buy paper for inkjet printers, if you have a laserjet then buy laserjet paper.
It's a great way to make lots of Disney themed t's for your hols, DD loved hers - courtesy of the Disign boards

- but they are not very long lasting (well mine don't last too long after a few washes!).