Wich subway station and how to buy the ticket for sub-way?

I will stay in Beijing Marriott Hotel. which subway station I have to go , if my itinerary:
and how to buy sub-way ticket, is it available in supermarket, grocery or only in sub-way station?
Day-1. SIlk Market (fake market) , then to Kungfu Show

Day-2. Tiananment Square, then Forbidden City

Day-3. Tempel of Heaven, then National Museum