ok, how do I get leather?
First, a butcher must slaughter or butcher an animal at a butcher's workshop; this will provide a skin. Then, a tanner will take the skin to a tanner's workshop and turn it into leather; this job is automatically queued once there is a skin close to a tanner's shop that isn't otherwise tasked.
To slaughter an animal, you enter the z screen, go to the Animals subscreen, find an animal you want to slaughter, and hit b. Alternatively, from the main screen scroll to the animal with the v-cursor, go to the p subscreen, and set them to be slaughtered there. Once you've done this, a slaughter animal job will automatically be queued at a butcher's workshop, and a dwarf with the drag the animal to the shop and slaughter it, causing it to instantly explode into its various meats, bones, organs, and other body parts.
To butcher an animal, first you must kill a non-sapient wild animal. Then, the carcass of that wild animal must be dragged to a refuse stockpile near a butcher's workshop, at which point a butcher job will automatically be queued at the butcher's shop. A butcher will come along and butcher the wild animal's carcass into its constituent parts. Unlike slaughtering an animal, butchering isn't instant. Also note that I use the term "wild animal" rather than just "animal" repeatedly; this is intentional and specific, as the bodies of dead stray animals cannot be butchered.
I remember something about selecting animals to be killed, but so far I've had issues with anyone dealing with aniamls (I just figured out how to work animal traps though)
Animal traps actually aren't related to leatherworking, as they cannot actually trap any animal that provides a skin. To capture wild animals alive, you need cage traps instead.
Also, in game time has almost been 2 years, and no attacks of goblins or anything (except for annoying goblin robbers, and stupid monkeys...)
Not unusual for someone starting out. Goblin ambushes will start appearing once you've got enough created wealth, exported wealth, or population, which you probably aren't experienced enough to develop quickly enough to have them showing up in the first couple years. If snatchers are coming, you'll get ambushed eventually.