i prefer storing photos on device, not in icloud. how can i do that?

If you don't want them on iCloud, don't turn on iCloud Photos. If it is already on and you want it off, see the instructions in Get help with iCloud Photos - Apple Support

If iCloud Photos is not turned on, photos will be included in an iCloud Backup (if you use iCloud Backup). You can exclude them from the backup if you want.