where and how to ride bus and go to terracota warrior,emperor mausoleum,and huaqing hot spring?

Yes, it can be finished in one day, but I prefer to suggest you find a local travel agency to arrange this for you. They can provide car and guide. Or you may spend much time on finding the bus from one place to another. If you want to go by taxi, yes, better to hire for a day. Talk about the price in advance. Both safe to ride bus or taxi, but I don't know if you can find where to take the bus. You can go to the bus stop in front of the Bell Tower hotel to Xian Railway Station. In the east Square (on your right side when you get off the bus and walk toward the railway station). There are green luxury bus to Terra Cotta Warriors. The bus total bus fare is less than 10RMB for single trip. If you take taxi, the cost should be 300RMB for a day.