How to open files with applications

Also how to perminantly set those extensions to open with the applications (unless of course you install other applications which change which extensions are associated with them...)

To make things simple you must get familiar with the properties of files, a file is simply anything that is not a folder (folders are yellow folder icons in microsoft windows operating systems), anyway i'll assume you know this part and move to the fun part:
Right click the file with extension which you want to permanently set to open with particular application