I was enjoying a nice day out on Saturday, so I'm still catching up with the fallout from this (and no doubt that will continue when I return to work tomorrow... 
).
Official advice from Microsoft (includes links to security updates for old systems):
Customer Guidance for WannaCrypt attacks
Blog by MalwareTech, the guy who inadvertently activated the malware's "kill switch":
How to Accidentally Stop a Global Cyber Attacks
It's (unsurprisingly) rather technical, but readable and interesting nonetheless. It also helps correct some of the media misreporting during all the panic and confusion. MalwareTech is not an amateur, but an information security professional: his job is to track and stop malware, thus assisting victims and law enforcement. He just happened to be on holiday at the time (his boss has given him another week off to make up for the lost break!).
Also, it looks like the "kill switch" wasn't actually an intentional "kill switch" (self-destruct mechanism) after all: current thinking is that it was a badly-engineered attempt by the bad guys to try to prevent the good guys analysing the malware. In theory, the bad guys' thinking was valid (and has been done before), but in practise its poor implementation provided the good guys with a simple way of stopping it altogether (even if they didn't initially realise it).