It is very important to RTFM to avoid fear, confusion, delirium, etc.
If you want to have fun mapping, avoid frustration, and achieve any level of success, sit down and spend plenty of time just learning TrenchBroom and getting a feel for how it works BEFORE you set out to make your epic map. If you aren't comfy enough with TrenchBroom before you try and build your masterpiece you're not gonna be able to easily translate the ideas in your head into a map and you will have a bad time. Start small and don't be overly ambitious until you know your way around TrenchBroom.
Follow the Wiki article for downloading and installing everything you'll need for Quake mapping.
Follow this Wiki article for configuring TrenchBroom for Quake mapping.
Follow this Wiki article. Note that many of the steps for opening a map will also be applicable when you create a new map, so it's worth reading. Plus, being able to open a map will let you test your TrenchBroom setup, especially your compilation settings.
Maps must be compiled beforet they can be played in Quake.
For common issues, see the Wiki article on troubleshooting.
- Trenchbroom Manual (Dense but packed with information, authoritative)