Allocating iCloud space, and clearing it out

I recently bought an iPhone, adding to my mac mini, iPad mini, and macbook - any of which might be making the issue here - and iCloud storage has suddenly got full. I'm assuming it's the phone, thought it's also possible that an OS update or general dunderheadedness on my part might have reset something. It's great having stuff on the cloud tho' i feel sure I've not needed it all till now [and have accounts on dropbox & google & a handful more places to put stuff already] and really don't feel like paying for any more...

[1] Where can I go to see everything that all my devices have put onto iCloud? I'm not even sure I know where to find this info on the separate devices themselves [I just found it on iPad...].

[2] When I know what's using up all the space, I will probably need to know how to control which apps do what on iCloud, though that's doubtless device specific so I'll wait till the first part is answered...