You use ago when you are referring to past time. For example, if something happened one year ago, it is one year since it happened. If it happened a long time ago, it is a long time since it happened.
双语例句
例： He was killed a few days ago in a skiing accident.
他几天前在一次滑雪事故中丧生。
例： The meeting is the first ever between the two sides since the war there began 14 years ago.
这是自从14年前双方爆发战争以来的第一次会晤。