Study statistics. --> Studying Statistics

Beginning on July.1 I have studied Mathmatical statistics And I attend a college class. --> Starting July 1st, I have to take a mathematical statistics at college. 

I didn't apply to THE class formarly so I hear the lecture more comfortalby. --> I am not officially registered in the class, so I will just attend and listen to the lectures (FYI we call this auditing a class) 

But Summer class is too short to catch up with all lecuters. --> But summer classes are short and it will be difficult to follow all the lectures (I think... I'm not 100% sure what you mean here). 

I have difficulty with studing statistics. --> This is ok but I would say, Stats is hard! 

I don't remember specific difficult method of differentiation and integration that I studied in high shcool. --> I don't remember the specific methods of differentiation and integration that I learned in high school. 

Accodingly I'm not accustomed with calculating complex problem. --> I'm not used to solving complex problems. 

Anyway, Because Statistics is required to study my major, I have to study hard. --> Anyway, since statistics is a requirement of my major, I'm going to give it my best shot. 

Good job. Best of luck with statistics... sounds difficult! I highlighted some areas you might want to double check in yellow. Be careful with verb tense and plurals. I also rewrote your sentences so you could see how I would go about writing about your topic.