Here at Hogwarts, there are seven mandatory courses for first years: Astronomy, Charms, Defense Against the Dark Arts, Herbology, History of Magic, Potions, and Transfiguration. They all have different levels of difficulty, explained more in depth in the chapters to follow. 

In order to complete first year, you must pass all of these courses nine lessons. After reading the lesson, you will complete the assignments (things like quizzes, midterm exams, exams, essays, and short-answers) and then be free to move on to the next lesson. There are nine lessons in each course. 

Grading here is different than in the muggle world: 

O's are a great goal. O stands for Outstanding, and it is the highest grade possible to get. 

E's are pretty good, too. They stand for Excellent, and fill a range from about

A stands for Acceptable. When it comes to taking OWL's in your fifth year, you will not be able to easily pass to NEWT level with an A. 

P stands for poor. It is the top of the fail grade range, much like the muggle D.

D is for dreadful. It is not a very good grade to have, for it means you are failing.

T is the absolute worst grade. It stands for Troll.  If you have a T in any of your classes, it would be a good thing to owl your teacher and try every extra credit opportunity possible in that class to bring it up to an O!