Since Liebe handled grammar so adeptly, I'll handle your questions.
My main problem is that I don't know when exactly to use the article such as "a/an the". Even though i know basic rule of grammar, i find it diffcult to use them in correct ways because of their diverse usage.
Yes, this is a problem for many learners of English. The only solutions are to (a) study the section on articles in your learning materials and also any online resources you can find; and (b) keep reading in English as much as possible so that, over time, the proper use of articles will come naturally to you.
Also, you might have noticed that most of my sentences start with something like "there are, she/he can, it is etc...". It is one of my weaknesses that I have shortcoming in using diverse grammar structures.
It's good that you see this and want to improve it. The strongest writing uses active verbs in a constellation of sentence structures. However, for the TOEFL, it is okay if you start sentences in these ways, so long as you do so correctly.
Lastly, i can't think of any ideas to write for my conclusion. Do you just restate what you have mentioned thoroughout your essay in your conclusion?
Writing in the real world, you may wish to use a diversity of strategies to conclude your essays. However, for the TOEFL you should stick to the formula of restating your thesis, summarizing your arguments, and closing with a memorable final thought.