Install XCode from the App store or the Apple developer website.
For installing XCode command line tools run the command
xcode-select --install
It'll prompt you to install the command line tools. Follow the instructions and now you have XCode and XCode command line tools both installed and running.
Open Terminal app (you can find it in Applications > Utilities folder) and run sudo gcc --version. This will prompt you to agree to Apple's XCode Terms and Conditions. Follow the on screen instructions.
If you run gcc --version again, and the installation was successful, you should see something similar to:
$ gcc--version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.0.0
Thread model: posix