Do I need a vapor barrier in my finished basement and if so how is it done?

I live in upstate NY. I am finishing my basement. I have noticed in the summer months that it is extreamly damp in the basement. I am uming I need a vapor barrier before I install insulation and finish the walls. Unfortunately the previous owners have already afixed 1x2's to the concreate walls so I cant paint on "dry lock" any ideas on how to go about this project. Any specific step by step instruction would be GREATLY appreciated.