First, head to the
Tremulous Mapping Guide and follow the instructions on how to install and configure GTKRadiant for trem. Most of us will tell you 1.5 sucks so starting out with 1.4 might save some time later on

Second, you can use
Google or any other search engine and look for Quake3 basic mapping tutorials, the site I ussually tell people to check out is
QWorkshop3. Tutorials are basically the same with a twist or two.
Taking for example the
First Room Tutorial, instead of using baseq3 textures you use the ones from the trem packs, which should be loaded if you followed instructions from the Trem. Mapping Guide correctly.
Next, Instead of using
info_player_deathmatch for spawnpoints as most q3 engine based games would, Tremulous
needs you to place a
team_alien_spawn and
team_human_spawn, these should be floating a couple of units above ground, you'll also need to place an
info_player_intermission for the spectator spawn.
To compile your map you'll have to either configure your GTKRadiant BSP menu (a guide for this is pending), use a frontend, or compile by command line. If you're on windows, you can use the Q3Map2Build frontend, I made a quick guide
here.
A few notes:
- If your map ends quickly with a "Humans win!" or "Aliens Win!" message right after loading it in tremulous, it means that there are either no spawns, or that they are touching or overlapping a brush.
- DON'T use CSG Substract or Hollow. Although it may seem like a usefull tool, it is also quite buggy, and often it causes more trouble than its worth.
- Use the Caulk shader (in textures/common/) on all surfaces that aren't seen by the player, its ussually better to just map with everything as caulk, then texture what can be seen.
- Read the manual, documentation is there for a reason, and theres a gazillion Q3 mapping tutorials on the net, google is your friend.
- Expect your first 3-4 maps to be crap, mapping is quite time consuming and while learning you often make a lot of mistakes, theres also a lot of ground to cover, even more so for those that want to learn to use the more advanced stuff (shader/particle scripting, hinting, etc). Just don't expect to finish a map within a week of making your first room, much less something playable.
- Soubok is also making a page with Tips and advices on Tremulous mapping, it can be found here.
I was planning on making a mapping101 page on the Mapping wiki, but I really haven't had the time (or the motivation) to do it yet.