Hi-
On any InfoPath control, dropdowns included, you can create rules, and these rules can have conditions, such as
my:dropdown = "Choice1"
So this means that the user selected Choice1 from the dropdown, and the actions you define within this rule will only execute if the condition is met.
We have free training available on this website if you'd like to familiarized with InfoPath:
http://www.infopathdev.com/blogs/jennifer/archive/2010/07/01/free-training-design-your-infopath-form-template-lab-1.aspx
Good luck!