The center of Xian is where the Bell Tower sits. Two of the city's main avenues converge there. They lead to the wall's entrance and exit gates. There are two more main avenues. The rest of the streets follow a grid pattern. Most streets are parallel to the main avenues. In the picture you can see the East Gate.
To move around you can take the inexpensive taxis, or the city public buses. You will know which bus to take by looking at the tourist map of Xian -- which is sold in streets in the train station area and around tourist sights.
All buses have a map of their route next to the entrance door. I recommend studying the map for a while before visiting the city of Xian. Note that the train station is always at the top of the maps, and the main Wall Gate at the bottom. If you take buses, watch out for pickpockets.