Questions

its easy to start.

you could go to your local rei and get a couple biners and some climing webbing, or you could order a kit online. i got my kit from freeflow lines website and couldnt be happier with the hand crank ratchet it came with.

umm as for advice, dont start by holding onto a tree or anything, it just throws off your balance. look at the end of the line as your slacking, looking at your feet will mess you up. use your arms for balance, but use the leg that you arent standing on more, it helps a lot. maybe listen to music too, if you zone out and dont focus so much on falling you'll have more fun and make it further.

just invite some friends to a park, bring a frisbee, and enjoy the day. it takes time to learn, just be persistant.

here is my take.
Where?
It is very easy to find somewhere to set up a line, trees in parks and yards are always nice, but i have setup lines on all kinds of things, like the feces. If you can't find some nice trees get creative with your location

How hard?
It takes a good bit of practice to get going, but once you get the hang of it, it isn't too hard.
learning to start off the ground, not hugging the tree, is hard at first, but will become natural.

What to know?
pick a point on the other side of the line, like a part of the tree or part of your tightening system etc. to focus on when you are walking i don't really know why, but i've always been told to do it like that and it helps.

Injuries?
The only bad injury i had was when i first started my brother sewed loops on the end of the anchors, they worked for a while, but one time i was sitting on the line bouncing and it snapped and the biner got me in the ankle, thought it was broken, bad all around.
But that was just dumb, other than that i maybe roll my ankle a little bit every once in a while.
And i have never nutted myself on the line, if you are falling off you will most likely fall to the side

Thanks for all the responses, I'm looking into buying something more geared towards starters. I'm really excited to start, it looks awesome.

buy gibbon, cheap, wide, and super easy setup!