Easy places to get a taxi, include the taxi stands at:
1. Rua de San Francisco, on the corner closest to Plaza Obradoiro, nearest the arch and Parador. this is at the part of the street where the raised sidewalk begins. This is considered "old town" and is very central to the places most pilgrims go.
2. Plaza de Galicia, just around the corner from the airport bus stop. Continue about 20 meters beyond the airport bus stop, around the corner to the right and you will see a taxi queue. This is considered "new town."
3. Estacion de tren - the train station. There is usually a taxi queue in front of the train station, if you happen to be in that part of "new town."
4. At the edge of "old town," where Avenida de Figueroa joins Rua do Pombal. This is at the edge of Alameda Park, where all the cafes are. Avda de Figueroa is the street the National Police Station is on. There are usually taxis here as well.
There are other places with taxi stands, but these are the most prominent, IMHO.
N.B. It starts to get scary when you know all this without a map...time to move...there...
I hope this helps.