I drink too much and want to stop but I don't know how?

I am trying to lose weight, but I drink...a lot. I drink a lot of beer too, which is the main reason I am gaining weight, instead of losing it. At the beginning of the week I told myself I wouldn't drink until the weekend..so far I've gotten drunk every single night. Last night I ordered vodka tonics because I figured that was better than beer, but I eventually ended up w/ the beer. I know the drinking is bad and I want to keep it for the weekend only..but its hard because I hang out with my friends and they're drinking. How can I keep myself from doing this to myself nightly?