Hi Ms, if you are located in the city center currently, it is suggested that you could take a city-bus or a taxi to the east square of the Xi'an Railway Station firstly. Here you could find the tour special line 5 and buses of Nos.914&915 drive you to teh terracotta warriors and horses directly.

The line 5 will take you about 40 minutes to reach the Terracotta Warriers, while the 914 & 915
The 914 & 915 will take you about 50-60 minutes.