Q4)I have an idea about manga story. I think it's good, but I don't know how to present it to publisher. And I need someone who draw manga based on my story. What should I do?
A)That's the same about manga.(Q1) Show your work to editors by sending it to publishers or Anime studios in your country. If you can go to the companies directly, it might be best. (Of course you have to make a phone and get an appointment befor that.)
Q3)About Manga tool, what kind of paint, pencil, etc. do you use for your drawings?
A)In Manga, we usually draw it with G-pen and drawing black ink. But I'm not sure you can get them in your country.(At least I heard that you can't get it in Germany) I think drawing pen will be OK. Draftsman's outfit is useful, I think. About painting, I always use Dr.Ph.Martin's color ink, but it maybe not so inexpensive. I think poster color will be good for beginner.
Q2)When I go to an publisher, am Isupposed to show them a finished manga or can I also show them some drawings or a finished part of a manga.
A)Maybe you should show them a finished one. If you are fresh, how can they know you have the ability to finish your long manga? And usually, in Japan, the pages which publisher give to a new face are only 30-40 pages. ( If your story is shorter than it, that's better. Because short story is easier to get a seat in Magazine, or for editor, it doesn't take long time to read.) If you couldn't make a good short story,  you also couldn't make a good long story, that's their thought.
Q1)I want to become a manga-ka. But I don't know what should I do. Give me an advice!
A)I don't know your country's situation, but maybe you should send your work to several publishers or Anime-studios.
You send your work to some manga-publishers of your country. If they are interested in your work, or think your work is
worth to publish, they'll contact you.

I know it isn't easy from my experience. I had to draw a lot of works before my debut. But if you really want to became a manga-ka, you might be able to overcome every difficulty. Good luck !