Doubt about raising skelletons.

Sorry I'm new in this game and I'm tottaly lost in this game but as I've read, necros can raise skelletons from dead monsters right? But when I kill a monster it just don't let me raise a skelleton.

First of all, do you have a corpse exploition skill (it will be call Animate Bone Horror or something similar)? You can't animate minions without a skill that lets you.
If you do, it's the enemy type that is stopping you animating. You cant raise minions from enemies such as elementals.

Yea I got Animate Bone Horror, well I killed monsters in catacombs, and in the most part of the 1st map.

The catacombs is mainly undead, so your problem is most likely that they are enemies that can't be animated from. Otherwise, make sure you're in range of the corpse.

Basically they have to leave an "exploitable" corpse. Only monsters with flesh do so.

What monsters do that? Because all corpses disappear after I killed a monster.

Heres a list of non-exploitable corpses:http://gw.gamewikis.org/wiki/Fleshy

And add pet corpses to that list as well now.

It sounds as though you are still in the tutorial area of the game (pre-searing). In the Catacombs (pre-searing) you can raise undead minions from the spiders and the devourers, but not from the other undead mobs. Outside the Catacombs, but still in pre-searing, IIRC you can raise undead from every monster except the ice and earth elementals. Also, all monster corpses will disappear after a brief time, so you have to use the skill before the corpse disappears on its own.

Once you get out of the tutorial area, check the list Khasar posted because you'll encounter a much wider variety of enemies.

Oh thanks, got it now.