Jur - firstly, welcome to the world of mountain biking.
Secondly, I to say it, but your (bike) frame will get the occasional scratch. It's just part of mountain biking. My frame has held up pretty well, but it does still have some paint missing - however, it's not the end of the world.
Thirdly, while some of the tracks are steep and quite tricky to ride, most of them are fairly achievable. You will feel uncomfortable on them, but with support & encouragement from us (hear that guys?? Be nice!
), you should get the hang of it pretty quickly. The firetracks are relatively smooth, however some have a fair amount of imbedded(sp?) rocks - but they are pretty easy to ride. The singletrack looks a little harder, but just take it easy and you should be fine (or walk - don't worry, I'll be doing that a bit