For is used to indicate a period of time.
- It rained for three days.
- I studied English for six years.
Since is used to refer to the starting point of actions or situations.
- They have been married since 2005.
- It has not rained since Monday.
Ago is used to say when past events happened, going back from today towards the past.
- I bought a bicycle four days ago.
- The company was founded 30 years ago.