You'll probably see Ieronimus Torres Medievales de la Catedral across town before you actually find this grand place of worship. Rest a while inside if it's open or just take in the facade from outside.
You'll find this principal church in Salamanca Centro. Once you've explored everything you want to at Ieronimus Torres Medievales de la Catedral, head to nearby New Cathedral of Salamanca, San Esteban Convent, and Art Nouveau and Art Deco Museum.