We use: tell + somebody.

I told David about it.
Did you tell him?
He told me that he was ill.

We use:say + something.

He said hello.
He said he was ill.

We use: say + something + to somebody.

I said hello to him

We use tell when we are giving facts or information:

tell somebody (that)...
tell somebody something
tell somebody about something
tell somebody who/when/where.

She told me (that) she was ill.
She told me the news.
Tell me about your new job.
He told me where it was.