I am new to Nintex workflow looking for where can i find information on how to do build a variable that querys a list ?

I am new to Nintex workflow,  I am trying to build a form that need to be routed according to 2 selections.  the first one is decides what part of the form needs to be filled in and will route the form only on 3 of the 4 selections. the second selection only applies to 1 of the 4 first selections and routes the form to a person base on a list of cities.  I have figured out the first part.  but I can't figure out how to build the variable and query the list for the second selection.  Where can I find information on how to do this?