The JR Yamanote line is operated by East Japan Railway and it is Tokyo's busiest and most celebrated train line.
It is a circular line which connects Tokyo's major city centers and it would take about 1 hour to travel the complete circuit.
[Tokyo Station to Shinjuku Station by Yamanote Line]
Take Yamanote line bound for Shibuya Station and it only takes 30 minutes.(200yen)
Here is Tokyo Station map and check station platform is number 5, Yamanote Line Outer Tracks for Shinagawa to get Shinjuku Station.
Trains run in both directions(inner outer), clockwise and anticlockwise, around the loop and there are departures about every two to four minutes in each direction.
The Yamanote loop through the center of Tokyo, connecting 29(30) stations including the Shibuya, Shinjuku, Ikebukuro, Ueno, Tokyo Station.
Staff members speaks English so, you can ask the JR EAST Travel Service Center in English.