Be honest. Tell them where you need help and what YOU will do in order to make a change and be more successful.
Kristina's suggestions are good ones.
In addition, you should probably be looking at other schools just in case your appeal is turned down. My suggestion is that you find a really good community college and enroll in their English remediation courses. From your writing above, you still need to work on lots of basic skills in English before you should be tackling university-level courses and all the researching and writing involved in them.
"I have received an exclusion from the university I have been attending because I failed 3 courses this semester. So the school asked me to write an appeal letter in order to get a chance to re-enroll. Please help me and give me some advice. Thanks a lot!!!"
I see issues with verb tenses, plurals of nouns, and run-on sentences. Here are some good places to study online, but learning from a real live teacher simply cannot be replaced.
Hold your cursor over STUFF FOR STUDENTS and choose what you want to work on.
Use the Grammar and Grammar/Vocabulary Quizzes (on the left).
Grammar quizzes -- to help you narrow down what you need to work on.
Lots to browse in here -- list on the left.
Lists of print, audio, and computer self-study materials you can purchase in stores or online.
Here is another point. If they exist, make sure that you include an extenuating circumstances that might have contributed to your failure and why the circumstances will no longer effect your future work.
Even so, the community college alternative is a good one, because you are more likely to receive individual attention (if you ask for it) and the cost is less.
I hope this helps a little more.