Wow.. I wouldn't have asked my buddy either.
The search function could've helped you, but oh well..
Square one: the 3 most common chokes are Improved cylinder, modified, and full. In that order, from least constriction, to most.

Take the current one out and see what it is (they unscrew...get a choke tube wrench). They're marked on their side. A guy ought to have at least an improved cylinder and modified to cover duck hunting. Once you have that taken care of, visit a printing store and get some big sheets of paper, about 4'x4' and draw a 30" circle on them. Shoot at the circle at different ranges with different shells and choke tubes, and count the hits in the circle. Take notes on each combination. Use the combination that gives a good looking pattern that's not too tight or spread out for the distance you're planning on shooting. Also take note of the densest part of the pattern. It might not be in the middle.

Don't forget,
Use anti-seize lubricant on your choke tube's threads. They can/will get stuck, or be a pain to remove the next time if you don't.

FWIW, I pretty much always use a modified on ducks even with close shots, and usually size 3 steel.