I'm going to assume you're working on a Mac -- that's what the 'M' in MAMP starts for (if you're on Windows you want WAMP)...
Can be downloaded from http://www.mamp.info/. Installation and configuration is simple, full documentation is on the MAMP site. You can put your site files within the MAMP application (htdocs folder) or point MAMP to your sites directory or a specific folder within. With a bit of extra configuration you can customise the address of your local development server and specify the local port that is used. Further information on this can be found at https://drupal.org/node/66187 - just skip all the Drupal stuff to lower down the page!