If you got the Ada mode as a separate distribution, you should have a look at the README file. It explains the basic steps necessary for a good installation of the emacs Ada mode.
Installing the Ada mode is basically just a matter of copying a few files into the Emacs library directories. Every time you open a file with a file extension of .ads or .adb, Emacs will automatically load and activate the Ada mode.
See the section Using non-standard file names if your files do not use these extensions and if you want Emacs to automatically start the Ada mode every time you edit an Ada file.
See also the Emacs documentation (emacs) for general usage variables that you might want to set.
This Ada mode works best with Emacs 20.3 or higher (the easy editing features for the project files won't work with any older version), but most of the commands should work with older versions too. Please try to install the most recent version of Emacs on your system before installing the Ada mode.
Although part of the Ada mode is compiler independent, the most advanced features are specific to the Gnat compiler http://www.gnat.com.
The following files are provided with the Ada mode distribution: