How do I disable iCloud on my iPhone? (having problems with contacts)

Ever since I've updated my iPhone to the iCloud, everything on my iPhone has been screwing up.

The most notable and annoying of these has been that I cannot do anything with my contacts

There is no option to edit them (the plus sign in the top right hand corner flashes on for half a second when I open up the contacts then disappears), I cannot add any new contacts, and I cannot delete old contacts that I don't want anymore on my phone.

I've had to physically write down people's numbers on a piece of paper that I have around with me at all times because I can't even add new contacts to my phone!

If anyone has a way to either disable iCloud on my iPhone or help me with this whole Contacts problem, I would be really grateful. 🙂