iphone storage

check what is taking all your iPhone storage space..

How to check the storage on your iPhone, iPad, and iPod touch - Apple Support

Here is some general info about storage

About storage on your device and in iCloud - Apple Support

Unless you've made some effort to back stuff up, its not likely that much is backed up. You can reference this link to see what is using iCloud

Manage your iCloud storage - Apple Support

But this still isn't exactly a backup, it's more of a sync. If you start deleting stuff, it will likely be deleting from iCloud as well.

If photos are taking up the majority of you space, you'd need to consider either buying iCloud storage and using iCloud Photos. iCloud Photos has an option to Optimize Storage, when you check the storage on your iPhone, there should be a recommendation that tells you approximately how much iPhone storage space you could save.

Buy more iCloud storage - Apple Support

Set up and use iCloud Photos - Apple Support

OR

you could export your photos and back em up elsewhere, then delete them from your phone.

Transfer photos and videos from your iPhone, iPad, or iPod touch - Apple Support

If you find that a bunch of your storage space is eaten up by 'other', see this link

Clear Other storage on your iPhone or iPad - Apple Support

Login to https://www.icloud.com/ and you’ll find there what are the stuffs that already backup by iCloud.

good luck!