Adobe's Fill and Sign is a really useful (free) product for filling in and signing documents. It works slightly differently on different platforms, so on this page, I've linked to a number of videos which should help you.

On a PC

On a PC Fill and Sign is part of the free Adobe Acrobat program. You'll need to download and install that. The following video describes how to use it.

On a Mac

Using it on a Mac is pretty similar to a PC, but here's a video specific to a Mac.

On an iPad / iPhone / Android