skordakias: 1) excellent story and plot
Planescape: Torment puts all other video game stories I have seen so far to deep shame. And yes, like StarEye said, everything else might feel a bit lame after that, especially since many fantasy settings have black-and-white morals and such. Of course, you could always start a new playthrough in Torment just to experience the story one more time. I do, and I haven't tired of it so far. If anything, I keep discovering new things and re-discovering brilliant stuff that I've forgotten.
skordakias: 2) interesting characters
Planescape: Torment says hello. It's not just the story that makes it great: the characters may seem weird if you just read quick summaries of them, which is why you should play the game and see how much more sense a floating skull makes then.
skordakias: 3) not too complicated controlling
That would be Planescape: Torment then. The battle system is pretty simple, but that's okay since you probably won't be using it all that much anyway. Most of the time you'll be just clicking around to move or to talk to someone. Roughly estimated, half the time you'll be actually playing the game, half the time reading what's going on. The writing is gripping, though, so don't worry. They really knew what they were doing.
skordakias: 1) killing hordes of zombies and mindless orcs. It is ok to kill a few, but just a few.
With stealth and cunning, I think you could play through Planescape: Torment without killing almost anything. Combat yields relatively few experience points, so there's no incentive from that front either. Just as well, considering the rather lacklustre combat system (compared to, say, Baldur's Gate or Icewind Dale). Some of the spells are fantastic, though.
skordakias: 2) a lot of special in game terms for attributes, potions, skills etc. It will be my first RPG so I have to understand what is going on.
You're in luck: Planescape: Torment has this one covered as well.
I may come across as some sort of a human advertisement for the game, but I honestly can't recommend Torment enough. There's a very good reason why it's often called the best game of all time, not least by myself.
Just for the sake of variety, I'm going to include another game as well. The Witcher is a pretty safe bet. In my opinion, the battle system is pathetically simple considering the amount of fighting (mindless bloodbaths, I'd guess, but often you can avoid them simply by outrunning your pursuers), but at least it's very easy to get the hang of it. The level-up system is a bit baffling at first, but you'll get used to it in no time: even if you don't it's pretty difficult to screw the game up with putting your points on bad skills. The story is pretty good as well. It's not the best RPG around, but I'll be damned if I regret having bought the Collector's Edition thing.
EDIT:
skordakias: Do I have the option to avoid combat during gameplay?
If not, will I have to fight regularly? (every 5 minutes? every hour?)
Just give me a rough estimation, if possible.
You can run from most enemies or sneak past them. The frequency of battles largely depends on how fast you read.
skordakias: All screenshots look quite dark. Is the whole game like that?
Quite. It's a very different sort of game for those who are used to Dungeons & Dragons in the Forgotten Realms sense. There are some open areas too, but most of the game takes place in Sigil, which is a city no matter how you look at it.