How to speed up my iMac after upgrading to Sierra?
Putting an SSD inside will give the most speed boost possible for general computing. If you have a specific benchmark or measurement you want to optimize, there might be other options. 

Since that’s an internal upgrade, you should consider what value you can recoup selling it used and buy a better / newer used Mac than trying to upgrade internal parts on most iMac. 

Since you have a hard drive, you will get the best performance with a hundred free GB and rebooting once a week or so. Other than that, watching for RAM hogs and keeping browser tabs under control are good tips as well.