What's Hammer and how do I get it?
Hammer is Valve's level creation tool packaged with all source engine games. For portal 2, you access it through the Portal 2 Authoring Tools which can be downloaded in the tools tab of your steam library. It's a complex tool that often misbehaves, so it can take a while to get used to. For some basic information to get you started, check out our wiki and the official Valve Developer's Wiki.
How do I make custom content?
There's a few different kinds of custom content.
Textures, the basic image/material combinations applied everywhere in the world, are created using VTFEdit. More information about that can be found here.
Models are any highly detailed discreet object, optionally with animation and physics. Valve has official plugins for Maya to export to Source's SMD format, which can be found in a download of the source filmmaker. There also exist plugins for other professional modelling environments to export to SMD. For more information, go here
Particles are small sprites or, rarely, models that move and sometimes interact with themselves or the environment. They are used when you need a lot of small, moving things, such as when a portal hits the wall and fails. More information about creating these can be found here
Note: this thread is intended to give more in-depth answers for more complex questions. If you have a simple question that can be answered quickly, direct it here
Use this thread to post your questions about Portal 2 mapping or to answer previously posted questions. The OP will be periodically updated with questions/answers.