You can look at this to see ways that you can manage iCloud storage. Please, for the time being, Pay no attention to the section about managing Photos in iCloud. I want to direct your attention to the section about which apps you can choose to backup to iCloud. If you are using iCloud Drive, you may have files that you can delete.
Manage your iCloud storage - Apple Support
If you are unsure about anything at all, then don’t do anything. Post back with questions.
When you plug your phone into charge, you are most likely launching iTunes which then will perform an automatic sync and it will backup the phone as well. There are two things that you can do to prevent the phone from backing up when you charge the phone.
- Charge the phone using the supplied power adapter and plug into a wall socket. Don’t use the Mac to charge. I am assuming that you are not set up to sync over WiFi when your phone is locked, plugged into power and iTunes is running. If you don’t leave iTunes running all the time, then you cannot be auto syncing over WiFi.
- Turn off auto sync in iTunes. Launch iTunes and at the top of the screen (in the menu) go to iTunes>Preferences>Devices. Click on the box at the bottom of the resulting window that says Prevent iPods, iPads and iPhones from syncing automatically. Click OK. You can quit iTunes after that. iTunes will no longer start up on its own when you connect the phone to your Mac.
If you turn off auto sync, you will have to initiate the sync process on your own when you connect to iTunes again. I have never synced automatically with my iOS devices. I always start the sync on my own.
Once again, if you are not comfortable doing this, don’t do anything at all. Post back with more questions.