East towards Xi'an
Take any of the buses 30, 34, 37, 38 from the train station or town center. They will stop very close at the motorway entrance in direction Xi'an. The road to the motorway is already a toll road, so don't leave the bus at the toll station, the 'real' motorway is about 8 km further.
Be aware that Tianshui basically consists of 2 cities lying about 10 km apart. You might have to take a bus or taxi, if you end up in the wrong part. The train station is in the Eastern part and there are some cheap hotels around it.