How to store my photos in iCloud

See this article:

Set up and use iCloud Photos - Apple Support

Take notice of this part if you want to save space on your iPhone:

Save space on your device

iCloud Photos keeps all your photos and videos in their original, high-resolution version. You can save space on your device when you turn on Optimise Storage.

On your iPhone, iPad or iPod touch:

1. Go to Settings > [your name] > iCloud.
2. Tap Photos.
3. Choose Optimise [device] Storage.

Good luck!

See the information in Set up and use iCloud Photos - Apple Support

Note that iCloud Photos is a syncing service. Any photo you delete from a device where iCloud Photos is turned on will also be deleted from iCloud and from any other device connected to iCloud Photos. You can potentially reduce the storage space required on a device by turning on Optimize Storage.

Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.