For the baby stages, writing various variations on "hello world" modules, and virtual hardware drivers, are the best way to start (real hardware introduces real world problems best faced when you have more of an idea what you are doing).
"Linux Device Drivers" is an excellent book and well worth starting with: http://lwn.net/Kernel/LDD3/
LDD (used to, at least) have exercises where you wrote virtual drivers, e.g. RAM disks, and virtual network devices.
subscribe to https://lkml.org/ or to the mailing list of a sub-system you will be hacking in. Lurk for a bit, scanning over threads, reading code review (replies to patches) to see what kind of things people stumble on or pick up on.
See if you can obtain (cheap) hardware for a device that is not yet supported, or not yet supported well. Good candidates are cheap-ish USB NICs or similar, low-cost USB peripherals. Something with an out-of-date, or out-of-tree driver, perhaps vendor written, perhaps for 2.4.x, is ideal, since you can start with something that works (sort-of), and gradually adapt it/rewrite it, testing as you go. My first driver attempt was for a Davicom DM9601 USB NIC. There was a 2.4-series vendor-written kernel driver that I slowly adapted to 2.6. (Note: the driver in mainline is not my driver, in the end someone else wrote one from scratch).
Another good way in is to look at the Kernel Newbies site, specifically the "kernel janitors" todo: http://kernelnewbies.org/KernelJanitors/Todo This is a list of tasks that a beginner should be able to tackle.