Since it is used with point of time, it means when something started.
E.g.: I haven’t seen Lucy since Holi. Tom has been waiting since morning. We had met several times since we were kids.
For: For is used for a period of time. David was at university for four years. Peter has been studying French for a month. David has been at university for six months.
Ago: Ago is used with the simple past tense.
Rohan and I met two weeks ago. I lived in Brussels 10 years ago.