however, as someone with a bit more age and experience, i can say that the people for whom marijuana tends to cause serious problems likely have been daily smokers for several years beforehand (here i'm talking about psychological problems rather than simply skipping class or missing work). you're not going to take a few puffs off a joint and become schizophrenic. BUT if you smoke a few joints every day for a few years, you very well may develop SERIOUS problems with anxiety, depression and social relationships. problems you didn't have before you started smoking. problems you likely will not have if you stop smoking.
so the people saying that weed's going to make you kill yourself or never be happy again are the exception. but the teenagers shouting "weed is harmless! you fucked yourself up!!" don't know as much as they think they do.