How do I delete the iCloud?

When you create an Apple ID (to use on an Apple device ... App Store, iTunes Store, etc.) an iCloud Storage of 5 GB is also simultaneously created for you for free and is associated with that Apple ID.

Depending on how you setup your Apple device and/or use your Apple ID/iCloud, data from your device(s) are uploaded and saved on iCloud. Note: it is not mandatory for you to save data on iCloud ... it is optional and up to you! If 5 GB is not sufficient you have the option to purchase more space and also to downgrade and stop paying for extra storage, etc.

So, strictly speaking, you cannot delete iCloud. But, yes you can certainly delete all or some of your data that is saved on iCloud.

Important Note: Again depending on your setup, how iCloud is connected to your device(s), be careful, if data is deleted from iCloud the same data will/can also be deleted from your device(s).

So, first, backup your device or save the data on your iPhone somewhere other than iCloud ... maybe on a computer or another cloud service.

How to back up your iPhone, iPad, and iPod touch - Apple Support

Archive or make copies of the information you store in iCloud - Apple Support

Second, verify your iCloud Settings ... turn OFF iCloud feature settings that you do not want to use and/or sign out of iCloud.

Change your iCloud feature settings - Apple Support

Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support

Only then, delete data from iCloud.

Manage your iCloud storage - Apple Support