If you need to ask where the bathroom is in Spanish you should say:

-Donde esta el baño? (Where is the bathroom?)

You can use other words for bathroom instead of 'baño' like:
-Donde esta el servicio?
-Donde esta el aseo?
Some vocabulary that might be useful is:
vater / inodoro (toilet)
bañera / pileta (Argentina) (bathtub)
lavabo (basin)