Eorl of Rohan
Most of the action is taking place in the vicinity of Minas Tirith.
The Encircling Mountains,
Echoriath, were in Beleriand which was destroyed in the first Age during the War of Wrath.
Here is a map of the area of Gondor.
The mountains you need to use are The White Mountains,
Ered Nimrais - they run from Gondor to Rohan.
Here is a reference for you to use to look at various topics and maps:
The Encyclopedia of Arda