What should i do to prevent of getting sick because of the high altitude going to lhasa?

Before going, you could have some helpful medicine, such as Rhodiola. It is suggested that you refer to these tips: https://www.travelchinaguide.com/essential/tibet/

we live in Canada and have just returned from our vacation to that high land(Sep 12 2011) Our Dr prescribed Diamox (or Acetazolamide) for altitude sickness prevention. They come in 250 mg pills. Two days prior to our train there, we took 1/2 tablet in the morning and 1/2 tablet at night, and continued this prescription for the duration of our journey until we we departed. We weaned ourselves off the meds by taking 1/2 tablet a day for two days afterwards.
Our side effects included tingling in the hands and feet. We did experience some headaches due to the altitude and blazing sun but Tylenol came to the rescue. Advice: drink lots of water!