Scripting

I'm trying figure out the scripting for a pet project I'm doing. I have five fields 4 are text and 1 is a drop down menu. I want to add 3 text fields together in the fourth field but I also want part of the calculation to be: if I select a certain thing from the drop down menu to add 10 any help would be awesome. I'm very new to this so take it easy lol.