I took iCloud: 50 GB Storage Plan but I dont know what it does and where to fine what I have backet up. I also have drop box.

iCloud is a syncing service that lets your share data among all your devices/computers.

If you want your data in iCloud, go to Settings/Your Name/iCloud, sign in, and then slide the switch on the right side until it is green for items you want in iCloud such as Photos, Contacts.....

Set up iCloud

Set up iCloud (2)

You can also backup your device to iCloud.

Backups – Choose Method

Back up your iPhone, iPad, and iPod touch

You can see what your iCloud storage is used for at Settings ➔ Apple ID(Top) ➔ iCloud ➔ Storage ➔ iCloud ➔ Manage Storage on your iOS device or System Preferences ➔ iCloud ➔ Manage...... on a Mac.