By subway, take the #1 local train to the 116th Street station, which is adjacent to the main entrance to the Columbia campus, or to the 125th Street Station. Alternately, take the #2 or #3 express train to the 96th Street station, and transfer to the local #1 train. (Note: after 96th Street, the #2 and #3 train travel to the east and away from the Columbia campus.)
For directions to the Morningside campus of Columbia University (car, train, bus, etc) and a map of the Morningside campus, see the page Visiting Columbia.
Google Street view