Randomly choose a line - algorithm

Question

We have a large file that can't fit into internal memory. How do we randomly pick one line so that each line has the same probability to be picked?

And how do we randomly pick such n lines so that they all have the same probability?

We don't know the number of lines beforehand.

Any hint on where to start solving this, which algorithm to use, or at least an idea where to start would be appreciated.