im plan to start my trip from Xi'an to Beijing

You can take a normal train at Xian Railway Station or a high-speed train at Xian North Railway Station to BJ West. Upon your arrival, you can take a shuttle bus to Military Museum. At this MTR Station, take local subway line 1 and get off at Tiananmen East or Tiananmen West Station. Then you can easily walk to Tiananmen Square for a visit. After that, you can walk to the nearby Forbidden City to enjoy.

To be honest, to cover all the destinations you mentioned within a day is impossible. Anyway, after Tiananmen Square, you can walk to the west square of it to take bus no. 120 or 2 to Tiantan Ximen Station (the West Gate of Temple of Heaven) and then have a visit there.

As for Great Wall, which section would you visit? If it is Badaling, you can take bus no. 919 or 880 at Deshengmen Station, near Jishuitan MTR Station to reach directly; if it is Mutianyu, at Dongzhimen Outer near DOngzhimen MTR Station, you can take bus no. 867 to reach directly.

I went to all the exact places it is really fun!! Anyway I think the easiest way to get there is by train! Being a child my self it was a great experience and i slept really well on the train!