It is only about 3 kilometers from Beijing Railway Station to Donganmen Street. So you can take a taxi, and it will cost you less than 15 Chinese yuan.

Besides, you can also take public transport such as subway and city bus.
By Subway: take Subway Line 2 at Beijing Railway Station and get off at Jianguomen (totally 1 station) and then transfer to Subway Line 1 and get off at Wangfujing (totally 2 stations). The price of the subway is 2 Chinese yuan regardless the distance and transfers between lines. Go out of the subway from the north exit and walk northwards along the Wangfujing Avenue for less than 10 minutes then you will find the Donganmen Street.

By city bus: take bus No. 420 at Beijing Railway Station East and get off at Sun Dong An Plaza.