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.
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).