Ago is followed by an expression of time.

The train went out ten minutes ago.
I met her two weeks ago.

Tenses with ago

Ago is normally used with a past tense because it refers to a finished time.

She phoned ten minutes ago. (NOT She has phoned ten minutes ago.)
‘Where is Jane?’ ‘She was in the garden ten minutes ago.’

Note that a present perfect tense can be used in the structure since…ago.

I haven’t bought anything since a week ago.
He has been working for us since about ten years ago.

Ago and for

Ago says how long before the present moment something happened. When used with a past tense, for says how long something lasted.

He died three years ago. (NOT He died for three years.)
He was ill for three years before he died. (= His illness lasted three years.)

Ago and before

Ago is used with a past tense and a time expression to count back from the present. Before is used with a past perfect tense to count back from a past moment.

I met her in Milan three years ago.
When we got talking, I found out that we had worked in the same office ten years before.

Before can also mean ‘at any time before now/then’. In this case it is used with a present or present perfect tense.

I have seen her before.