From which railway station in Xian will the bus for Terra-Cota leave?

It is from XA Railway Station and yes, it is the one close to the City Wall.
There are conductors on the bus, which will come to your seat to charge.

But there´s also a bus to the terra-cotta from the north railway station, isn´t it? and is there a possibility to luggage storage in the north railway station? regards, karin

Yes, there is bus service from the north station to Terracotta directly and that is for free. To take that, you just need to show their train tickets on the arrival day and the entrance tickets to any attraction in Lintong including Terracotta (the tickets for thse attractions are available at the Tourists Service Center on the underground floor of the railway station).

And yes, there are luggage-left counters in the north station.