My trip to china approaching. First stop is Beijing. What should I visit first and should I book it in advance?

Firstly, thanks to everyone who answered my last post. You guys have been extremely helpful. Thank you.

Now I am traveling mid march and I decided to not only extend my trip but visit more cities. So the plan is Beijing/Zangjia/Shanghai/Guangzhou/Back to Beijing. Each will last 3 nights except Guangzhou which will be 5.

What should I do first in Beijing?