Hi
You need on your computer an application that can read and write using the form's file format.
E.g., If the form is in .doc format you can use Microsoft Word.
Most forms are sent in pdf format, alas the most common the pdf reader does not let you type in.
If this the case you can download install and use this free reader.
http://www.foxitsoftware.com/pdf/reader/reader4.php
Otherwise print the form, fill it in by hand, scan it to as a file, and send back the scanned file.