How to learn a lot of Japanese in little time

Hi guys,
Please guys help me, I'm in big problem.
First of all, it all started when my school did a project about reading and wanted us (students who know different languages) to participate by reading something in that language. I did it because I could speak a bit of Japanese but then everyone thought I'm fluent in Japanese so I thought that I have to increase my vocabulary and a bit of grammar so please can you help me.
How can I increase my vocabulary and from where)
How can I know what is the grammar that I really need when travelling to Japan (I know grammar of n5, n4 and some of n3) is it enough(!
How can I learn a lot in short time)
I know there is no magical method that would make me fluent, but at least, something like a daily plan.
Note: I don't need Kanji. My vocabulary is poor in all languages other than my mother language.
Thank you guys in advance.

From your post, it's a bit hard to figure out what you need, but I'm going to take a crack at it. Here's the thing, if your assignment is only to read something in Japanese, then you are over-panicking. One of the biggest mistake language learners make is thinking they need to learn an entire language all at once in order to communicate or read. You don't. Cramming won't help you learn a language. It might help you through your assignment, but I wouldn't recommend cramming if you are actually interested in learning.

Point 1: I used to be a fluent French speaker. Yes, used to be. I can still read it and sound like a pro--even if I have no clue what each and every word means (I have a concept, but no literal translation to give). You don't need to know an entire language to read something and pronounce it correctly if you pick the right reading material.

Point 2: I didn't learn grammar from any of my teachers in school--I hated learning English in school. They made the subject super boring. I learned it from reading actual books. I learned my verbose vocabulary from reading--again, not school. So, if you want to up your grammar levels and your vocab at the same time, the easiest way is by reading books and these days getting your hands on children's books in furigana is way easier than it used to. Because why can't you use a story that is written in furigana to read to your class? They aren't going to know the difference and there isn't any reason why you can't show them all the book and explain the differences in the characters. Once they get the gist of how hard it is to learn reading kanji, they'll be impressed you can read at all!

Please look at http://www.fluentu.com/japanese/blog/easy-japanese-books/. The author gives all kinds stories at all kinds of reading levels. Yes, there is some manga so you'll need to skip those probably for the purposes of your assignment--unless you just really want to read them. I would recommend 'Stories you can read smoothly' for easier reading or 'Sadako and the Paper Cranes' if you are further along. There are lots of great suggestions on the blog. By the way, I am in no way affiliated with said blog or any of the material on it. I am just an avid reader who is trying to learn how to read Japanese as my fourth language.

Your assignment isn't to be a fluent Japanese speaker. It is to read a story and sound Japanese. Do yourself a favor: it's ok to admit you are not fluent, but that you are learning. It's ok to admit that Japanese is a difficult language and that youth in Japan are still learning how to read the more complicated characters in high school! I hope I don't sound pretentious or too presumptuous, but if you really need help, you'll take the above advice and read a children's story. If you don't want to listen to my advice and you are stuck on the idea of learning Japanese really fast Good learning materials include Human Japanese (an app) , Rosetta Stone (for vocab--not so hot on grammar), anki (another app which you can use to learn kanji), and many more if you look out on google. I haven't found a great resource on grammar yet, so I can't speak to that.

Thank you so much. I think you are right, reading is the best way for everything I need. I will try doing so.
Thank you again!!