It's more of an Apache skill versus a Linux one. Does your server have a control panel? Or are you managing your sites by hand. The reason I ask is because most of the commercial control panels out there allow for you to make adjustments like this from the web versus having to find the file. Essentially, you are going to be looking for your httpd.conf file on your server and will be making a change for your particular site. You are going to need to change the "AllowOverride None" for your particular site to "AllowOverride All". Your best friend here will be the Apache documentation.