First off find what's consuming the space on your imac
Download and use
OmniDiskSweeper. It will provide a sorted list of what's consuming your space.
Secondly determine what in that list can be moved to an external drive. For me, using a MBP, I had my iTunes library on the external drive (that was something in the 50GB range), and I set up my Lightroom library in such a way that prior years were on an external drive but the current year was on my local drive. I also ran my VMs off the external drive as well.
Everyones different, but you may find that a lot of your data can be housed on an external drive. The only other piece of advice is backups. Don't forget to backup your external drive.