You use ‘ago’ to say how much time before now something happened. You are counting back from today.
This whale lived millions of years ago.

You use ‘since’ to refer to the starting point of time that has passed.
I have been waiting for the train since seven o’clock.

You use ‘for’ to say how long something lasts.
You can stay underwater for about forty minutes when scuba diving.
(photo from Tom Horton)