I'm correctly playing an assassin, sort of.
First of all, how do I stealth kill? I've got 2 daggers however when I come behind the enemy there's no stealth kill animation, all I do is attack...
I'm a dagger/bow character and I just met my first dragon. Now seriously am I always going to be such a [censored] against dragons? All I could do was basically shoot him with my bow while he was attacking some soldiers. I'm lvl 7 on expert diff...

I'd like to know the same thing. Maybe you have to unlock stealth kills as a perk?

There is no stealth kill. If your backstab does enough damage to kill, you might get an animation. If it doesn't do enough to kill, you do a normal attack for extra damage.

If all you're doing is two daggers with the base backstab damage, it's doubtful you'll be killing in one shot on lots of enemies.

If you're sneaking (crouch) and you attack while undetected you get a damage bonus. This bonus can be increased through perks. You have to be sneaking and undetected to get the damage bonus.

Wait so I better wield one dagger ?
I rly am thinking between playing as an assassin or starting a battlemage, feels like he will be stronger .
Have u guys got to a pretty high level till now with an assassin, maybe u can give some expressions, if u like him or not?

No, I have no idea where you got that. You need to put perks in stealth and use better weapons. At least until you perk the 15x for daggers.