How do I upload my photos onto my iCloud account?

When photo library is set up in Mac https://support.apple.com/en-in/HT204264

And its preferences are set https://support.apple.com/en-in/guide/photos/pht5156cc968/4.0/mac/10.14

See the article https://support.apple.com/en-in/guide/icloud/mmb5dbaf376f/1.0/icloud/1.0

If a photo is literally on the desktop screen of Mac , open photo application just drag and drop the desktop photo inside the application .

It will be also uploaded in iCloud.com , the reason Mac is signed with Apple ID and password , the photo box is turned as on and syncing with iCloud https://support.apple.com/en-in/guide/icloud/mmb1de40d9/icloud

Read the link https://support.apple.com/en-in/mac-backup

If you enable iCloud Photos in Photos for Mac, the Photos Library will be synced to iCloud automatically. The version in iCloud will always be identical to your system Photos library. But I am not sure bout what you want to achieve. The title of you question is asking about "upload to iCloud", the question itself is asking "backup my computer". Do you want an offsite storage or a backup, or both?

iCloud is a syncing service, not a backup. You can store your documents and data and your Photos Library in iCloud and keep it identical and perfectly in sync across all your devices. iCloud will be the main storage of everything, and your devices will be working with local shadow copies, mirrored from iCloud. But Cloud will not be a backup, as you are working with the items in iCloud. if you accidentally damage a file or delete it, this will sync to iCloud and you cannot use iCloud to recover the files. use iCloud, if you want a central storage with easy access from all your devices, but you will still need a backup to protect you from data loss and user errors.