I'll recommend a read-through of the manual, and also the Arcanum subforum here on GOG.

One thing in particular to pay attention to is the "adversarial" or juxtaposed relationship between mechanical and magical elements, since this has long term effects on your character and party of NPCs, and sometimes immediate effects during gameplay depending on what you're trying to do at the time. This isn't to say there is a right or wrong way about it, but it's helpful to understand this part of the game world early on since it'll help you avoid some of those "Why in the world isn't this working?" moments.

> it's a good game?
Yes.

> what's the story (no spoilers, just a summary)
Ancient myths become modern information warfare.
Initial character hook: You have some vague clues of a deeper mystery - uncover it so that other people stop bothering you.

> is the gameplay good?
Sort of, but make sure to patch the game. The most annoying "feature" is NPC AI that can't be switched off.

> is there character development?
Yes.

> how many hours is the game?
The reviews say 30 to 40. That's about right for a first playthrough if you don't chase sidequests (but you should).

> how many playthroughts do I need to make to uncover every possibility?
About 4 to see the important plot branches (if you know where to look); more for gimmick characters and even more to see most NPC interactions.

> do you get an epic feeling while playing? does your choises influence in the story?
HELL yes.

> how many races and classes are in the game?
8 playable races for male characters, 4 for female characters. The system is skill-based, so classes per se; you can be a weapon-using combat monster, magic combat monster, employ various interesting tactics or just loot the enemies when your NPCs are done. Three fighters and a healer (thus, minimum 2 NPC slots) is enough to kill anyone.

it's a good game?
-hell yes

what's the story (no spoilers, just a summary)
-saving the world and the clash of technology and magick

is the gameplay good?
-as long as you play turn based

is there character development?
-yes

how many hours is the game?
-many

how many playthroughts do I need to make to uncover every possibility?
-2 or 3, dunno

do you get an epic feeling while playing?
-yes

does your choises influence in the story?
-yes

how many races and classes are in the game?
-human, elf, half elf, dwarf, gnome, halfling, half orc, half ogre

it's a good game?
No.
what's the story (no spoilers, just a summary)
You do stuff to find out stuff.
is the gameplay good?
Yes
is there character development?
it IS an RPG, what do you expect:
how many hours is the game?
I played it ... like ... 2 weeks?
do you get an epic feeling while playing?
Yes, but I feel epic writing this post. I'm ALWAYS epic.
does your choises influence in the story?
As in Fallout
how many races and classes are in the game?
you can play an idiot.

Even though I haven't played the game, I'd say go for it! I'm sure you won't be disappointed, and even if you were, you could always play something else instead!

I bet it is a great game because if it wasn't, I would be mightily surprised!

@OP:

Please,please, please read the manual. I know it's a basic thing but this game REQUIRES it :D.

And stay away from the real time combat it's ...wonky.