Hi,
A very basic explanation is that you tell information but you say words. eg
Mary told Tom her name.
Tom said, 'I'm sorry, I didn't hear what you said. Please say it again'.
It can get more difficult than this, and there are plenty of contexts where it's not so clear, but that's the basic concept. If you want to write and post some sentences using said/told, we can comment on them for you.
Best wishes, Clive