You should wear your winter wear there, it is cold in Beijing. You need to get newest information of Beijing Weather(https://www.travelchinaguide.com/climate/beijing.htm) before going there. You can pack according to the weather forecast.

For Beijing Attractions(https://www.travelchinaguide.com/attraction/beijing/) ,the must sees are Great Wall, Forbidden City, Tian'anmen Square, Temple of Heaven, Bird Nest, Water Cube, well, you should check Beijing Tours(https://www.travelchinaguide.com/package/beijing.htm) for itineraries and destinations suggested.

Enjoy your time!