Not much, video calls use a lot but I think they only work on wifi? They used to and I haven't tried them since.
I have the same plan. The thing that uses the most is email, the little icon on your phone? If you leave it logged in it constantly uses data to refresh itself and you will go over.
If you use any data using app, you need to double tap the home key and then the apps running on your phone will pop up, hold your finger down on one until they all show the x, tap the x to close them all so they aren't still using data once your done using them. This also helps with battery.
I don't have any specific numbers for you, but most apps don't use too much. Videos do. Like the Netflix app, pandora does and running email does.