Knowing a few words of Mongolian can help immensely to break the ice when meeting locals, so get familiar with the basics:
Hello sain bain uu?
Thank you bayarlaala
My name is... minii ner ...
Locals won't expect newcomers to know much more than this, but every bit helps.
In downtown Ulaanbaatar many people (professionals, students, hotel workers etc) can speak some English, especially if they are under age 35. In other parts of the city and in the countryside it can be more difficult to find an English speaker. There are ways to get around this, such as having a phrasebook or an app on your phone.