Need more information:
1. What exact iMac model do you have?
2. What exact version of El Capitan are you running?
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 those tasks. Two of the best (that I use) are Onyx and Tech Tool Pro.
4. Are you making backups to an external device? That is
so, so critical!
5. What third party applications (ie, non-Apple) are you using? Are each of them compatible with El Capitan? This site can help you with that:
https://roaringapps.com/apps
6. How much free space do you have on your internal hard drive?
I wonder if you need to Repair Permissions. You can do that with Onyx, available from here:
https://www.titanium-software.fr/en/onyx.html
Make sure you get V3.1.9, the one that is compatible with El Capitan. When you download it, install it, and launch it, one of the "tabs" will say "Maintenance". Click on that, and you'll see one that says "Permissions". Click on that, and run a few times. You might also consider running the other two tasks (Scripts, and Rebuilding).