Since and Ago are often confused and used wrongly by learners of English in the TEFL class. But there are some simple rules which show how they should be used.
The usage is quite simple: since is followed by a specific time.
since + specific time
I’ve been waiting here since 3 o’clock.
I’ve had this toothache since yesterday morning.
I’ve lived here since 1991.
Ago comes after a period of time.
period of time + ago
He arrived five minutes ago.
I bought this car three months ago.
The war between the two tribes started 100 years ago.
Note that ago refers to a time which started in the past and is still going on now.
Finally, never use the two words in the same clause.
* I bought this car since many years ago.
* An asterisk denotes a grammatically wrong sentence.
I bought this car many years ago.
I’ve had this car since 1998.