I must admit it can be intimidating at first.
I joined from finding a thread on reclaiming PPI (yes really!) under a different username about 3-4 years ago and I recall once I posted something I think about Breton t-shirts and an opinion and I got jumped on like you wouldn't believe and almost left - after that the other posters admitted they'd either been harsh or some other MNers told me others could be harsh and to ignore it.
I don't post much on AIBU or reply to it… actually I've found it less 'ragey' recently - safer boards are Chat, The Litter Tray (if you have a cat!), S&B, Relationships are good.
what is amazing about MN is that if you really are being a bitch or nasty to someone you soon get pulled up over it, either on the boards or by a message so I think most people tend to try to be nice. OK you'll get the odd home truth, someone who disagrees with you but not everyone you'll gel with, that's life.
If you see a friendly poster/username you like the sound of when you see their posts reply or join in. some people here are sadly a bit hostile to newcomers so there is nothing wrong at all in posting that you are new here. 
also remember that there's a lot of gossip and rumour-mongering sometimes, par for the course really - either ignore or indulge.
like with most of the internet and chat rooms don't take it too seriously.