Unable to back up icloud

Go to settings then to iCloud. Scroll down and tap storage and backup. Once you're there, you're able to tap on previous backups and then toggle off applications or whatever else you don't necessarily want to backup to iCloud.

with Icloud you get 5GB free storage

iphone comes in 16GB, 32GB and 64GB

if you use more then 1/3 of your storage on the iphone least least storage you're full

you can buy more on a rent base with apple or you can backup to a computer