I just want to add that whatever you are doing it's working! Your writing has continually improved since I met you and what you wrote above, you couldn't have written earlier. There were just a few tiny things to correct. You did a great job writing in this discussion. Reading is a wonderful way to learn. If you read enough, when you write something it will just "seem right". Keep up the good work and like Rachel said, short sentences are fine. If you can form good short sentences, little by little you will be able to write longer ones well too. 

Don't sweat it. You're doing great! :D

It's a good question. I also struggle with writing, but here are some suggestions.

1) Write. You have 24 notebook entries, which is a good start. After you receive corrections, try to review the grammar that you missed, and memorize the vocabulary.

2) Perfect/practice your grammar. Have you at least completed a comprehensive english grammar course? I believe that systematically reviewing grammar, doing drills and such, in conjunction with writing mentioned above, is quite helpful.

3) Increase/reinforce your vocabulary. When you encounter words you don't know, or when you have to look up a word to properly express yourself, memorize that word. Make sure you understand what context it can be used in. It's good to memorize the word <em>and</em> a sample sentence.

4) Have a balanced learning plan. I can't emphasize this enough. You should be doing all the skills, because they all reinforce eachother. Reading, writing, listening, conversing, grammar, vocabulary and pronunciation. I'm not suggesting to spend equal time on all of these skills, but you shouldn't neglect any of them.

The Complete Plain Words was written by a British civil servant as a prescriptive writing manual for government officials. It is very detailed indeed and covers just about everything you could possibly want to know about expressing yourself accurately and clearly in English.

Strunk and White is the timeless American classic written in 1919 by an English professor at Cornell. It has the benefit of being very short and so quick to read. This might be the best place to start because of that. However if you are learning to write British English then there are some differences in punctuation, spelling and style you need to be aware of. The Trask book can help you understand those.

You will probably refer back to these books continually. I've been writing English for 45 years and not a week goes by without me needing to use them. The most important thing though is to write, get it checked and learn from your mistakes.

The more you read, the better you write. Books written about good English style are trying to explain difficult concepts and put forward arguments, which seems to be what you want to do. These books have been prepared with the utmost care to avoid embarrassment and so they are excellent models to follow. Reading them will do you no harm at all even if you do find them difficult.

Thanks very much for your valuable advice and attention! As I had an emergency a few days ago and needed to spend several days handling it, I have not read all your comments until now; it is so delightful to read some more new comments and advice for this discussion.
Yes, Dorothy, you are right! I often make some minor mistakes in my writing, like those appeared in the foregoing comments. I need to be more careful to avoid those errors; maybe I need to check my writing after finishing it. Besides, like you suggested, I need to read more to learn some new vocabulary, master how to use them in my writing and speaking and make my writing more understandable and interesting. In reality, right now I still find sometimes it is difficult to choose some right words to convey what I am thinking and what I want to express. I think only by reading more can I solve this problem.
Thanks for your excellent suggestions lan! I in fact have read the book called ‘the Elements of Style’, a 52 pages book, as I remember, containing some basic knowledge about grammar, rhetoric devices, and so on. I believe that I have understood most of the ideas and thoughts the author related in that book, but I did not get much from it, probably because my English level as a non-native speaker’s is not good enough then to absorb them. However, I found the book ‘The complete Plain Words’ by Ernest Gowers, and I am reading it when I am available recently.
Also, besides usage of vocabulary, mastering idioms and phrasal verbs seem to be significant to English learning, since in most native writing material I often read a large measure of idiom expressions and phrasal verbs.

I agree with most of the comments others have made, particularly Rachel's observation on long sentences. Even native English speakers write sentences that are really hard to understand simply because they are long and poorly constructed. It would be very interesting to see examples of the type of test questions you will face. Maybe you could post some?

Even the most complex arguments and concepts may be expressed in short sentences. They would make your arguments more compelling and the concepts more readily understood. You might find these free resources at the Plain English Campaign useful: http://www.plainenglish.co.uk/free-guides.html.

It also helps to become aware of common errors in written English. There are several books that might help you in this regard. Mind the Gaffe (Trask), The Complete Plain Words (Gowers) and The Elements of Style (Strunk and White) are all excellent.

Mind the Gaffe is both an amazing reference resource and a book to dip into just to discover something new. It was written by an American linguist who worked in a British university. Consequently it contains lots of useful information on the differences between American and British written English.