How do I mirror my hard drive?
Generally speaking to "mirror" a hard drive often means setting up a RAID where one drive mirrors the other. This is done using either the RAID software built into Disk Utility or 3rd party RAID software.

The benefits of that is that each drive is a duplicate of the other and if one fails the other disk has all your valuable data. The "problem" is that you generally cant set up RAID across a USB or Thunderbolt bus (that I am aware of). So if you have a Mac with one internal drive you can't set up a RAID between the internal and an external drive.

You can also use a utility like SuperDuper, Carbon Copy Cloner (and others) to make a bootable duplicate of your HD, essentially making a mirror copy of the drive, just not done in real time like a RAID mirror would.

Both of those applications can be scheduled to run on a regular basis (hourly, daily, weekly, etc.) and copy only the changed files to the backup drive.

I use SuperDuper to do just this and it is scheduled to run every other day automatically. It connects to another Mac that is always on, mounts a disk image and backs up the changed files.