iCloud - backup and removal of saved items / iMessages on iPhone

[bufoqusi]

I've an iPhone and an iMac and backup to iCloud each time I'm prompted but recently I've been notified my 5GB storage capacity has reached its limit. I've googled unsuccessfully answers to some questions so can anyone help please (1) is everything saved on my phone & Mac automatically saved to the iCloud at each backup, and if so, does that mean that if I want to save things but not to the iCloud that I have to save them on say an external drive? (2) is it possible to set my phone & Mac so that only certain things are backed up, and if so, how do I do this please? (3) can I save iPhone text threads away from my phone please, and if so, how is this done please? Grateful for some answers in layman's terms please. Thank you