How should I buy an admission ticket to Xian City Wall?

I wanna watch Warrior Parade Performance (Barbican) and Relieving Guard Ceremony and explore the Wall in the morning and the light show at night at South Gate. How should I buy the admission ticket to the Wall? Is there any specially-priced ticket for senior foreign citizen?

Do you have any a-few-day pass to all attractions in the city? If so, what is the price?