Welcome to mapping! It's a heck of a lot of fun.
I started just over a month ago, and am currently working on my first map that I intend to release (couple more months though).
I was in your position not long ago, and what really helped me was reading the SDK wiki (someone has already provided you a link), as well as picking up a copy of "Half Life 2 mods for Dummies". I've never bought a "dummies" book before, but this one really kick started my mapping technique. Most of the techniques can be used in TF2, however, some will be different.
Youtube has a number of video tutorials as well.
And of course, one of the best places to get advice is, well, right here! The people on these forums are so helpful and friendly - unlike most internet forums.
Good luck to you!