If there isn't an assassin job then just get a friend to make one up for you e.g. There's a tavern owner in *insert-town-name-here*, go and kill him etc. etc.
Ok. When you come into young adulthood and have been doing evil deeds since the beginning ( mostly giving the guy the warrants and not the guard ) you will come into a lawless bowerstone and you will meet up with the same guy you gave the warrants to and he will thus send you to the assassin's first quest giver.