This page will help you get started with mapping and level design. This first section will list the thing's you'll be required to download and provide a little information on setup and things to pay attention to. If you have questions, use Google. Google is your friend. You can find the answer to pretty much anything. Also, please take advantage of the large selection of tutorials here on JKHub.
(This is all for idtech engine games, if you want to map for other games such as games on the unreal engine this will be partly useless)
What you need to download:
GTKRadiant: The preferred version is 1.4.0 over the newer versions such as 1.5.0 & 1.6.x, and most tutorials are based off of this version. It's more stable and has various features which make 1.5.0 inferior for building. 1.6.x releases are pretty much the same as 1.4.0, but they are very new. So they aren't as stable yet.
GTKRadiant 1.4.0 comes with an older version of q3map2! Download the updated version and follow the instructions on that page to have it fully updated.
Pakscape: Assets in the Quake3 engine are stored in archives in the pk3 format. Pakscape allows you to easily create new PK3's, or modify existing ones. All your files need to be stored in PK3's in the base folder in order for the game to use them.
The Jedi Academy SDK: This is the source code for the multiplayer engine, plus various tools used in the design process. Specifically when it comes to mapping you'll be using BehaveD and EffectsEd a lot. They are both included in this SDK under the tools folder. There are a number of other useful tools included such as Assimilate/Carcass (compiling glm for models), and MD3 view for viewing MD3 models and their animations. ShaderED is included but if you need to make your own shaders, learn how to do it with notepad. ShaderED is extremely outdated and will do you more harm than good.
Q3map2GUI: This is an EXTREMELY useful tool created by Darth Norman. q3map2 is the command line compiler for maps. This GUI makes it so you can easily edit the compile options from an interface instead of painstakingly editing command lines yourself. This tutorial explains how to use it.
EasyGen: EasyGen is a program that can generate an entire terrain for you entirely from an 8-bit bitmap. Once it generates the terrain you can easily paint textures onto it and then export an alphamap+metashader to provide a seemlessly blended terrain.
GIMP: You will eventually find the need to modify textures, or make your own. In order to do so you need an efficient image editor. The ideal image editor is Adobe Photoshop, but that can be expensive and most people would rather have a free alternative. This is where GIMP comes in. GIMP has a variety of user friendly tools for the beginner to accomplish much. There are various tutorials out there on the internet on how to use GIMP, making it even easier to learn.
Blender: Blender is an open source, free modeling program. The UI is kind of confusing at first and it isn't as efficient at UVUnwraping as 3DS or Maya, but it get's the job done. If you want to make fully UV mapped MD3 map_objects you'll need this program. You can also create your own character models if you wish, but that is advanced work. There are a HUNDREDS of tutorials out there for 3D modeling in Blender. YouTube is a great source for many of them. 3D modeling is hard, but when you get the hang of it, it becomes an invaluable tool in your arsenal. mrwonko has quite a few < a href="http://www.youtube.com/user/mrwonko/videos">videos to get you started. He even shows the modeling process of making and UVunwrapping/texturing a sword. NOTE: If you download Blender, you will need mrwonko's Jedi Academy Plugin Suite. It's the only way to work with Jedi Academy's files (.md3, .map, .ase).