-If embarking in a full mountain biome, your essentially screwed because there is no water and no wood?
You can find underground rivers or pools to muddy some ground for food production. It's not impossible, even in frozen climates.
-Finding a perfect embarking spot seems so hard it pops my brains out, there is either no flux stone, no usefull ores to use with flux stone, no sand, no lava or an aquifer and so on son. Am I overprioritising some things here or is it supposed to be that hard
I think it's important to remember that this game is supposed to be
hard.
-I tend to abandon my fortresses after migration wave 5 since then ill have more then 20 idelers somewere and it feels incredibly messy -> startover feels better for some reason. Is this a normal for a beginner or should I just /ignore the idlers and keep at it?
If you're having a lot of idlers, no worries. It might behoove you to turn off the economy, though, or have lots of really cheap bedrooms. I don't have much of a problem with idlers when I assign them to military, fortress, and royal guards.
-How many deaths are acceptable during a siege and the likes. Managing an army is a risky business as it is (ouch hai injured dwarfs) but having to bury some more after a siege plus strange mood madness (got messages of dead crafterdwarfs, only to realise they were not slaughtered by goblins but by a crafter who went strange mood berserk during the siege 4 floors down in my fortress) makes things very frustrating. Losing 3 of my 4 diggers/masons most of the time makes me alt ctrl del -> end task in a fit of rage.
Try not to lose any! In my forts, especially early-on, if the military must join in hand to hand combat, something has already gone wrong. Use traps and entrance design to foil sieges without actually having to fight.
-it seems to take forever to haul stone, even if i dedicate 10 haulers with soely stone hauling on they take so long befor they fill up a stone storage thats only 5 floors away from the quarry area.
Don't store it in stockpiles. Create a zone
for garbage dumping, then dump the stone (d-b-d) in it. It will still take a while, but at least that's more space efficient.
-how dangerous is terrifying surroundings? really I don't dare to attempt in fear ill be overrun just after ive built some farms and crafter shops just to find my dwarfs dead.
It really depends on the map. Some are deathtraps, others are just mildly annoying. The type of wildlife in terrifying biomes has a big influence. Skeletal whales = run for your life. Zombies = dangerous, but not scary.
-Can things like skeletonzombiemonsters actually be tamed?
I don't think so.
How does someone build into the ocean?
There are a variety of methods, from draining the ocean first to dropping in magma to form obsidian, then mining upwards from underneath the ocean floor. You just have to take your own map into account.
I see Doomshifter answered, too, so I'll cut off here.