Two methods for you:
1. when you arrive at the railway station, please follow the directions to take the subway Line 4 and then get off at Xidan station. Then, you could transfer to subway Line 1 here and get off at the next stop at the Tiananmen Square, where the Forbidden City is located. The subway fare is CNY2/adult.

2. at the railway station, please follow the direction to the square, where you could take the free bus to the Bus Terminal of Beijing South Railway Station. Here, you could take the city-bus 20, which has stops at the Tiananmen Square, too. The bus fare is CNY1/adult.