Where's the Line?

Ok so one thing that I think is going to be one of my most difficult challenges on my practicum is the whole authority, don't be there friend, be a hardass thing. I would say that generally I am a pretty happy-go-lucky person but I keep hearing that you have to be super stern and strict and pretty much mean with the students at first if you want respect. They say that you can let off later on but if you start slack you're screwed. Only I'm not sure if I could teach for the first few weeks without any humour and super mono-tone. And honestly I don't think that's even good teaching as there is a good chance I would put myself to sleep! I like joking around and playing games. So what is the proper plan of action. Should I make up this alter-ego serious Izzy for the first few weeks or can I crack some humour out the first day? Because I also know that with us being younger and new the students might think that they can either get away with more or that we are in fact their friend. And don't get me wrong I would like my students to like me and whatnot but at the same time I don't want them acting around me like they do their friends i.e. swearing, talking about drinking and drugs, etc. which actually brings up another point. What are we supposed to do if a students tells you they smoked week on the weekend? Ya it's against the law, but you have no proof and it's off school property and not during school time so are we supposed to do anything? Report them to a councellor? Phone home?