Note: Paseo de San Antonio is the closest station, but it is currently CLOSED for renovations.
The closest open station is "Santa Clara." From the Santa Clara station, you'll want to walk south one block to San Fernando, and then walk east 2-3 blocks to 4th street. At the corner of San Fernando and 4th is the Dr. Martin Luther King Library, which is the northwest corner of the campus.
Without knowing which station you are boarding at, I really can't tell you what train to take, but if you look at the light rail map on the station platform, it should be pretty self-explanatory. Trains are identified by their terminal stations (either Mountain View, Winchester, Alum Rock, or Santa Teresa).
Also, keep in mind that light rail is not always the best option... there's other transit options that may get you there faster (Caltrain, DASH, express busses, etc.). Again, not knowing where you're coming from, I don't know if this applies.