The latest version of this software is available from: https://docs.metasploit.com/docs/using-metasploit/getting-started/nightly-installers.html
Information about setting up a development environment can be found at: https://docs.metasploit.com/docs/development/get-started/setting-up-a-metasploit-development-environment.html
Generally, you should use the free installer, which contains all of the dependencies and will get you up and running with a few clicks. See the Dev Environment Setup if you'd like to deal with dependencies on your own.
Metasploit can do all sorts of things. The first thing you'll want to do is start msfconsole, but after that, you'll probably be best served by reading Metasploit Unleashed, the great community resources, or take a look at the Using Metasploit page on the documentation website.
See the Dev Environment Setup guide on GitHub, which will walk you through the whole process from installing all the dependencies, to cloning the repository, and finally to submitting a pull request. For slightly more information, see Contributing.