Umm, when you installed your forum, you had to update the permissions on the files. Full Read/Write/Execute for each directory and full Read/Write for each file is suggested so that you can edit files from the forum itself...
Really, if you don't understand the lingo and you don't have faith in your ability to do things perfectly without a hitch, you should really consider hiring out a developer to do the job for you as things can go wrong (especially with v7.3 as a hurtle, I've had several issues in the past with upgrading through that version which have required me manually having to run through the upgrade steps).
