I would love to answer in Bulgarian, but gogo will kill me - although Katran will understand every word I say here.
should I master armor lore or constitution first and which of them increases survivability later on?
-> both do. So, master constitution first, followed by armor lore. If you invested the skill points properly, by level 75 you should have 3 masteries - bargaining, pyro lore, pyro focus, followed by constitution at level 78. On level 93 you should master armor lore, and then it's all a matter of common sense.
I was wondering is around which level do the damage mitigation items appear
-> glacial defender is available from level 15 upwards. Armors with damage mitigation based on armor lore mastery start popping up from level 75 upwards.
What the hell does the +%damage mod. really do? How and which kind of damage does it affect?
-> + %damage means + all kinds of damage (phys/fire/ice/magic/poison), and as such, it is a great thing to socket, especially if you have an access to a niob smith.
Looking at your post I can offer these pointers:
-> buy as much + all skills as you can, complemented by chance to block: close combat or chance to block
-> farm bosses (griffon/boar/gar'collosus/octagolamus/kral/scorpion, etc.) for set and unique items that will complement the build or have the most sockets
As soon as you get decent equipment boosted by enough + all skills, the going will become a lot easier.
And welcome to the FDM. It is nice to see another Bulgarian around