How to get data for a field from another doctype?

I have 2 doctypes Reservation and Base pricing with fields Reservation{Resource, Adult_count, Price_charged} and Base Pricing{Adult_count, Per_person_rate, Resource}
while making the reservation it should fetch the data from Base pricing and should give the total cost of Price_charged based on Adult_count and resource type and it should be different for weekends and weekdays with some calculations.
how can I edit the custom script, Can anyone help me out for solving this?