How do you react to a teenager who tells you stuff?

Well if youre sure her parents wont help the situation call Child Protection services. Thats very dangerous for a young girl to be doing. Her male friends could drug and take advantage of her or she could get so drunk she gets alcohol poison. There is a reason the drinking age is 21!

She could be hoping you will help her.

I think it depends on your relationship to her. When my step sister was a teen she drank ALOT and what I said to her is not the same as what I would do now that my own son is a teenager if one of his friends came to me and said he was drinking.

Try to offer her a safety net, tell her that if she is ever in a bad situation, too drunk, drunk driver, boy pushing her to drink more\ have sex, ect. she can call you. One time only, free and clear, no confrontation with the folks. If she calls, she wants help! Let her sleep it off then get her the number for a teen AA group, protective services, any community outreach counseling that is available.
If she does not call, she may have been trying to shock you or testing to see if you would run to report back to her parents. Teenage girls are hard to help even if they want help.

I agree...she could be asking for help. The drinking and unhappiness could (not 100%, but could) mean something is not right in her life. I would keep myself open to further conversations with you in case she wishes to eventually share what might be making her feel this way. You just never know what might be going on behind closed doors that is making her so sad. And even if it is nothing so bad as that...maybe she needs a listening ear. That bond that you may be able to form with her by remaining open might be very helpful to her in the future. We all need people to lean on and maybe she doesn't feel like she has anyone like this in her life. Not that that would be necessarily true, but maybe that's how she feels that it is.

If she is telling you these things then she is probably hoping that you will help her in some way call social services or talk to her parents if they wont help her the get the cops involved do what ever you can to help this kid she is going to really get hurt if she keeps up this distructive behavior

Help her.

if u dont care enough to consider some1 in need a responsibility then u shouldnt care at all, but a good person should see that obviously her parents suck as parents and she needs an actual adult figure to TELL! her what is right and wrong!
you keep asking "what can i do"
let her know that what she is doing is wrong and can ruin her adult life,
tell her that you don't approve and never let her do it around you,
tell her to talk to a teacher or conselour at school if she is having problems
she might not be your responsiability but she is asking you for help and you are turning your back.

What is your relationship with this girl? Maybe she is asking for help, maybe is asking for your approval, maybe she is testing you. It depends on your relationship with the girl and her family. How often is she drinking. I was a good kid and I started drinking occasionally when I was about that age - never drunk and only maybe a couple of times a year. I never had sex or did anything I didn't want to do.