What is different about the job compared to what you expected it to be? In other words, did you think the job would be a certain way and now it turns out to be different? Did you take the job because you wanted it, or because it was the first offer you got?
In almost any job, there is something you can learn to make you better in the future. If you decide to stay, look for how to develop skills you can use in later jobs.
But if you want to quit, it's pretty easy. Find the boss and say "I'm giving my two weeks notice, I'm quitting."