Will Backing up Iphone Erase Everything?

Restoring an iPhone from backup will wipe all its contents, then replace everything with what's in the backup. Whatever data is on your iPhone now, but not in the backup, will be GONE after the restoration process.

How do I backup my iPhone without losing everything?

Click the iPhone/iPad button at the top left of the iTunes window, then go to Summary > Back Up Now. Encrypt your backup by clicking Encrypt local backup and setting a password. Encrypting allows you to back up your Health and Activity data, as well.

Does backing up your phone delete anything?

Erase after backing up

After you back up, you can reset your phone by erasing everything on it.