Hey, at least she told you - now it's time to talk about it. Ask her why she waited to tell you and why she felt the need to try it in the first place. My guess is she knows it wasn't the right thing to do, she's bothered by it and she really wants you to tell her NO. My stepsons are the same age and have been caught twice by their mom for smoking pot without telling her anything about it. Their excuse is that 90% of the kids in their school smoke it (according to them) and when they are with their friends, there's always a joint being passed around, so they just go along with it.
When I was 12 and hitting puberty, my mother made this big speech about how if I ever needed to talk to her about sex, or birth control, or whatever, I could come to her. Then I was 18, had my first real boyfriend, and was thinking about going on birth control because I was thinking about having sex, I came to her and told her and she totally freaked out. You can be sure I never talked to her about it again (and ended up going on my own to the student health care center where I was in college and getting on the pill on my own). So whatever you do, remain calm, make it clear that you are disappointed she felt the need to still try it, but you are glad she decided to still come to you. She might be looking for you to tell her what she needs to do now. Tell her what your expectations are and make it clear what the consequences will be if she chooses to not listen.
If she just told this to you 10 minutes ago, then get off the computer, and go have a heart-to-heart with her. Tell her you are glad she was honest with you but she cannot do this ever again - she needs to understand the potential consequences of continuing to smoke pot. For one thing, t's illegal and if she gets caught by someone other than you, there could be ramifications as far as driver's licenses and college scholarships. It may be addictive and lead to other drug use (I know others may disagree). And for me personally, every person I've ever known who is a habitual pot smoker has not gotten very far in life - it's like they have no ambition because they are just constantly overly mellowed-out on pot and end up not achieving anything. They don't go to college, they work dead-end jobs, and live in crappy places (or are still living with their parents). Her having some concrete goals in mind (college, career, etc.) might help if she realizes that smoking pot won't get her closer to those goals. Also, underage drinking and experimenting with sex could potentially screw up her future as well. I also would be paying closer attention to who she is hanging out with and what they are up to since sometimes at this age they can still be strongly influenced by their peers and less by their parents (as seems to be the case with my stepsons).
My DD is only 3 but I am already telling her "Good choices cause good things to happen and bad choices cause bad things to happen." Your daughter needs to start taking responsibility for the choices she makes and how her life turns out from here on.