For

We use for when we are talking about the duration of an action or state:

• I have lived in London for seven years. (This tells us how long I have lived in London)

For is a preposition here.

Since

We use since when we are talking about when the action or state started:

• I have lived in London since 1997. (This tells us when I started living in London.)

Since can be a preposition (since five o'clock) or a conjunction (since I met her).