In order to boot into target disk mode, you need to connect two computers with a Thunderbolt cable.
On the target computer (the one you want to get data FROM),
- Boot into EFI by holding down the option key on startup and type in the EFI password.
- Then hold down the t key until the FireWire icon comes up.
From there, the host computer should be able to transfer files to and from the target computer using Finder in Mac OS or Explorer in Windows.
A more detailed set of instructions can be found on Apple's support page.