What if I buy some from the wrong guy? What specifically are you worried about? I always recommend getting a small scale that weighs to .1g so you can double-check that you got the weight you paid for.
What if I have to do a random piss test at work, like if I get injured (this has never happened)? That's a chance you take.
What if I get a new job and have to take a piss test? That's a chance you take.
Will I get addicted? I suppose you could become dependent on it - but as far as the scary "addiction" shit you see on TV where it takes over your life and you can't function like a human, then no, that's not likely at all. I've been smoking for 13 years now, and occasionally, I'll stop smoking for a month or so just so I stop eating everything in the house.
Is all this stress and anxiety over-blown, well placed, or worth it?! It depends on the strain you're smoking, as well as how your body reacts to it. Some strains give me too much anxiety and it's not worth it to me - other strains I can smoke all day and be just fine.
Is it really not as bad as beer? In what regard? No one has died from a cannabis overdose, whereas you can't say the same for beer.
What is the best way to do it? I prefer to vaporize mine, so I'm not inhaling a ton of burnt plant material, but that's jut my preference. However you decide to do it, start small, wait, then decide if you need more. Like, if you're smoking out of a bowl, take one small toke, then wait before proceeding. I don't recommend you start with edibles or concentrates - I'd definitely start with flower in a glass bowl or vaporizer.
How do I know I'm getting good stuff? You'll be high, you won't have a headache, harsh cough, etc.
How do I know I'm getting stuff that isn't laced with PCP or something else?!!?!?! PCP and other drugs are too expensive, no one is going to waste it lacing someones cannabis. Be more worried about mold, pesticides, etc.
What if I'm getting ripped off? Don't buy frm that person again.
What if I'm high and I get pulled over? Don't drive while high. The effects will generally wear off in a few hours. Note though, that THC and the compounds that it breaks down into in your body can stay for about up to a month after you consume cannabis. While you obviously wont be impaired, you could still fail a test. If your in a traffic accident or something and get tested, they'll say marijuana was a factor even if there's only a trace amount in your system.