Mybb is among the BEST forum scripts out there in the internet. It's open source (free) which means you can get started in no time. It is very light script which is great since it can support more traffic without putting too much stress on the server. It comes with the great feature that allows you to create your own plugins and hook them to the main core file. This is great since we don't need to modify core files in order to made some modifications to the script. It also comes with a great collection of free templates/themes you can choose from.
It's very easy to start simply upload all files to your server root directory. The installation is a simple as completing a few steps.