I want to completely format my computer and replace my current OS with Linux. How should I go about doing this? Particularly, how do I handle finding all the device drivers I'll need? I'm using a Lenovo ThinkPad T400
Thinkpads are very popular with Linux users, so there's a lot of documentation out there. The standard resource for Thinkpad users is ThinkWiki.
It's quite likely that a standard distro install will be sufficient. The Linux kernel probably includes all the drivers you need. Possible exceptions are the graphics drivers (look at the Nvidia or ATI websites for those) and in rare cases the wifi drivers.
Take the safest road: Try CDlives of some distros to verify that everything is recognized. If succesful you can install to your hard disk without worrying.
There is a lot of info on the Thinkpad T400, Linux certified sells a T400 with Linux pre-installed, your choice of Ubuntu or Fedora. Gentoo has a wiki for Thinkpads and so does Arch Linux. I think you should have a good experience with the T400. I would do as oposit suggests try a few live CDs, see what works and what you like and have fun :)
Be aware that recent thinkpads have a hidden partition (predesktop area) including all sorts of tools for system recovery and the like which can be accessed by hitting the blue Thinkvantage button during boot up. Installing Grub into MBR breaks this. With recent thinkpads you can install grub into the first sectors of the partition holding /boot and set this active, then you normally enter grub when booting but thinkvantage this works. I'm not 100% sure this already works with T400 but I think it does. See this thinkwiki page for more information.
I have Ubuntu 10.04 installed on a T410 and all drivers were in the distro. I did make sure to get the model with Intel video and wireless chips though, as these are well supported.