First off, Metamaps runs on Ruby On Rails. You'll need to get Ruby and Rails installed on your computer if you don't already have it. Go to here for Ruby http://rubyinstaller.org/downloads/
It uses postgreSQL 9.2 as a database. You can install that for your computer from here: http://www.enterprisedb.com/products-services-training/pgdownload . During installation you can choose whatever database password you like. Make sure to note it down!
Navigate to the folder that you want to download the metamaps files to and run the following: (use your forked git repository address if it's different than this repo. You will also need to go to your Github account settings and add the SSH key that was placed in your clipboard earlier)
1) Copy /config/database.yml.default and rename the copy to /config/database.yml then edit database.yml with your text editor and set the password to whatever you chose when you set up the PostGres database. Then do the same for /config/secrets.yml (the defaults should be OK for this file).