Hello I am coming from Beijing Youth Politics College and I'm wondering how do we get to The Water Cube water park by subway,

From your college, you need to take a taxi to Sanyuanqiao Station, where you can take the subway line 10 to Beitucheng Station. Then change to Olympic Sport Center Station. Get out of the station from Exit B1, you can walk to the Water Cube easily.
You can use the same method to go back to your school.