how do I backup iMac to iCloud

As mentioned, iCloud is not a backup system for Macs and Mac OS. The backup system Apple provides for Mac OS is Time Machine, this requires an external HD to backup to and Apple explains how to get started (after starting it's pretty automatic) in Back up your Mac with Time Machine. Many experienced uses on these forums prefer the OWC Mercury Elite Pro due to its reasonable costs, high reliability and the support provided by the supplier. If you are in North America, just click the link to the drive and you can buy it there. If you are outside N. America then you can still buy them however you would need to go through a distributor which is easy, you can start by clicking OWC International Resellers & Distributors to begin finding one in your general area.

It is NOT wise to use an on-line backup plan for Mac OS due to the amount of data being backed up. This means that they are okay for restoring small amounts of data but if you are ever in the position where you need to restore your whole system or migrate from an old Mac to a new one, they simply don't work! Also, it is wise to have redundant backups just in case one fails. Many many experienced users use this strategy due to learning the hard way, myself included. So if you want to use Time Machine as your primary backup and use an online backup service as your secondary backup plan that would be okay just keep them both going. In the event one fails, you have the peace of mind you are still backing up until (the next day or so) you can get your secondary backup working again.

Apple offers iCloud Backup for iPhones and iPads but not for Mac computers.

It is possible to sync data from a Mac to iCloud if you turn on those options (e.g., iCloud Photos, iCloud Drive including the Desktop & Documents option). Syncing has some of the benefits of a backup (and the benefit of being offsite) but is not the same as a backup. Major differences include the fact that not everything on your Mac can be synced and the fact that if you delete something, accidentally or otherwise, there is only a limited time period (30 days in most cases) during which it can be recovered.

Also see the Apple article: How to back up your Mac - Official Apple Support

And more information on syncing in:

Set up and use iCloud Photos - Apple Support

and

Set up iCloud Drive on all your devices - Apple Support

and

Add your Desktop and Documents files to iCloud Drive - Apple Support

Yes, it is very wise to have a Time Machine backup of the old machine, you can use it to migrate to the new machine per the directions below:

When you buy a new Mac the easiest, quickest and most reliable method to migrate from the old Mac to the new Mac is to simply connect the old Mac's Time Machine External Hard Disk to the new Mac using a USB cable before you turn on the new Mac. You will likely need a USB A to USB C adapter, you can find them on Amazon for $10 or less. When you start the new Mac for the first time it will startup Setup Assistant (SA), SA will get to a point where it asks if you are migrating from another computer, select Yes  and follow the on-screen prompts and it's done! It is THAT simple.

If you have already done SA on the new computer then simply connect the Time Machine External Hard Disk using a USB cable and launch Migration Assistant (MA) which is located in Applications - Utilities - MA. Then follow the on-screen prompts. 

The ONLY difference between MA and SA is MA will create a new user account that you need to log into.

Both SA and MA migrate applications (not 32 bit apps), data files & settings. They do not migrate Mac OS itself. Some professional apps such as Adobe and Microsoft apps may require re-installing their apps as they will not allow them to be migrated. As long as you kept the security keys for installing their apps, it is not an issue. 

Finally, never EVER use Wi-Fi when doing Setup Assistant or Migration Assistant. Why, it simply fails in most cases. Please read and follow the directions in Setup Assistant and Migration Assistant. If you do not have a Time Machine backup of the older Mac then create one! If you are not aware of Time Machine, please read Back up your Mac with Time Machine and finally if you need an External Hard Disk to use as a Time Machine External Hard Disk then many experience users on these forums prefer the OWC Mercury Elite Pro due to it's reasonable price, high level of quality and ease of use. 

By "backup my Mac" do you mean a full disk copy backup or just syncing iCloud-based apps (Photos, Passwords & Keychain, Contacts, etc.)?

If you mean the former, then Apple does not provide a method to do so. The latter; however, is enabled via System Settings > Apple ID > iCloud

Coyote_8_12_1953 wrote:

I'm purchasing a new iMac and felt a complete HD backup would be a good idea before transferring the data.

Yes, it would. If you make an up-to-date backup on an external drive, you can attach that drive to the new Mac and migrate using the backup on that drive. This is preferable to migrating over Wi-Fi and might also be easier than migrating using a cable connection.

The backup can be either a Time Machine backup, or a bootable clone backup (a.k.a. "startup disk") made with Carbon Copy Cloner or SuperDuper!.

I sometimes do a "quick and dirty" backup of documents by copying them to iCloud Drive. (I think the Mac treats "drag and drop" in this case as a move – so I copy and paste to keep it from moving/deleting the originals on the local drive.). But my main backups are on external drives.

There are some third-party cloud providers that provide backup services. For instance, I've seen people mention BackBlaze ( https://www.backblaze.com/cloud-backup ).