Nanning is over 600km from Guangzhou. You can travel by train, 12 hours, or by bus which is faster, about 8/9 hours. Luxtury bus ticket price is similar to the train ticket for sleeper. You can get to find the bus at Fangxun Bus Station and Guangdong Provincial Bus Station. My suggestion is go a bit early to the bus station if you do not choose railway. Then you may get to Nanning early at night.