How do I make my iMac Apache server always run?
so that I can access my home files when I am outside my home


If this is all you really want to do then you are using the wrong tool. 

Go to System Preferences -> Sharing and enable File Sharing.
Then go to Options (same panel) and check the AFP box.
Then forward port 548 on your router.

You can now access your files from the Finder ( Go -> connect to server) with read-write access instead of download only.