How to upload my photos to iCloud so I can free up my phone space

First you need enough iCloud space for all your photos. (The free 5GB will probably not be enough)

Then enable iCloud for photos in iCloud settings. Settings>your name>photos.

If you want to save the space on your phone, choose optimise iPhone storage in the same settings page. Photos will keep small previews on the phone taking up much less space. DO NOT delete these. That will also delete the full size photos out of iCloud.

After making the settings changes, it will take some time for photos to move the images from your phone to icloud.

If you delete a photo from the device the iCloud copy is also removed.