The thing with english exams, I found, is they asked you dumb questions, 'how did Steinbeck make Of Mice and Men more dramatic by using linguistic devices?' is an example. That doesn't make sense to me, so I sucked at english exams. However if you go and ask the f**king teacher how to get your head around the questions, you will be able to write something decent.
As for the quality of your english spelling and grammar, you should've been learning that from primary school. In the back of your head you probably know what you're doing wrong, so listen to that, and change it, take some time to do this.