3. Have you ever done any disk cleanup/maintenance/repairs, from a software perspective? You can actually do quite a lot of disk cleanup on your own, and there are some excellent products available (both free and commercial) that can help you with that
4. This is critical: are you making backups to an external device? If (hopefully) so, what software are you using for that?
I believe you can boot your Mac to the (hidden) Recovery HD partition, and get the help you need. This link explains how to do that, and what that partition can do: