There are a few ways, all of which involve plugging the phone into the Mac. Helium will back up all your apps and app data. If your contacts were entered as Google contacts (not Phone, Device or SIM contacts) they're already backed up at
Google Contacts (which is why all contacts should be entered as Google contacts). When you put your Google account back into the phone (you'll have to after a factory reset), the contacts will restore. (If you check the Sync settings under Settings/General/Accounts/<your Google account>, you'll see what can be automatically backed up.)
You can also use
Kies - Windows and MacOS to back things up. And you can copy any files from the phone to the Mac - just drag and drop, the same way you do to copy any file from "A" to "B", dragging them from the phone (it should show up as a drive) to wherever you want them on the Mac..
If you have any irreplaceable picture, audio or video files (baby's first steps, etc.), copy them by hand to your computer and to a cloud account, to be sure, so that even if the backup goes bad, you don't lose them. In backing up, there's no such thing as paranoia.
BTW, backing up a phone that has a lot of apps and files
will take a long time. Start the process about an hour before you go to sleep, so you can catch any mistakes, and go to sleep knowing that the process is proceeding. It may be finished by the time you wake up.
