So you've got a fresh, clean Mountain Lion install and you need to get up and running for local development. Recently I spent a day doing just that, so I thought I'd write it all down, to save me from having to look all this stuff up again.
Install Xcode Command Line Tools
Available from the Apple Developer downloads area. You'll need an Apple ID.
This package includes llvm-gcc, which is a (mostly) compatible replacement for gcc. It will work for most homebrew formulas which build with gcc.
homebrew can be conveniently installed with one line in the Terminal. As with all curl-to-sh style install scripts, you should read the script first to make sure it's not going to do anything bad. Don't say I didn't warn you :)