Ago vs. Since
Both words speak of the past, and they are often used interchangeably.
Ago - from the present to the past. It is used after the word or phrase it modifies, especially with the simple past tense, not with the perfect tense.
e.g - It happened a few minutes ago.
Since - from the past to the present. It is used with the present or past perfect tense.
e.g - I haven't eaten since breakfast.