Where to start depends a lot on what you want to do.
If you want to be a sponge (and there is nothing wrong with wanting to learn as much as you can about something new) and soak up all the information you can, here are some good basic references.
About dot com's intro to Linux
. It's oriented to Ubuntu, but the information is solid.
. It's Slackware-oriented, so it's very good on basics, such as permissions, file structure, and the like.
Garrels's Introduction to Linux
. Extremely detailed and very thorough. I read it all the way through and have a copy on my bookshelf.
If you are looking at Linux as a user, don't get too distracted by detailed descriptions of esoteric commands. Learn the commands you need, especially the Linux equivalents of common DOS/Windows commands; the rest you can learn as you need them.
Welcome to Linux. It's different from Windows; there is a learning curve, and I have realized that, ultimately, it's easier and more reliable than Windows.
And there's no registry. That alone is worth the ride.