At this point, it's time to get yourself a copy of the current Linux kernel source and -- I'm going to make a long story short -- avoid every other option and simply check out the main Linux Git repository anywhere under your home directory:
$ git clone \ git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git \ linux-2.6
The other options typically include downloading a fixed tarball or installing a distro-specific package but, for maximum flexibility, you really want your own Git clone, even though it's going to cost you some bandwidth. So pick a location, start the clone and go get a coffee.
And while I'll be coming back shortly to show you a few useful Git commands you can use later, you don't need them just yet so I'll put those on hold so we can head into the lesson you've all been waiting for ... how to configure, build and boot your new kernel.
NOTE: If you just want to browse the current kernel development tree, you can find it here.