I hope to go to china to visit great wall and forbidden city. So I've decided to stay 161 hostel in

Your hotel is about 3 kilometers away from the south gate of the Forbidden City, so you can take a taxi to get to the Forbidden City.
Which section of the Great Wall in Beijing would you want to go? Anyway, if you want to go to Badaling, you can take a taxi to get to the Beihai Bus Station and then change to Bus No. 5 to get to the Deshengmen Bus Station where you can take the Bus No. 919 to get to the Badaling Great Wall directly.