If you want to create a manga, then there are several things you should do before even starting to write the base story.

1. Preparations
You need to set your mind on one theme and then make some research. My manga, for example, includes demons and angels. So I had to study some myths and some characters before I decided what to have in the manga. You also have to think about the period of time that your manga will take place in; number of characters; places; etc...

2. Orientating your story
The thing that I find most helpful is to make a TIMELINE of certain events. So let's say that we have a vertical long line. Put a little line on the beginning and on the end to emphasize the beginning and end of story. Write what happens in the beginning and how the story ends.

Now, cut the line in half. this is the peek of the storyline. Once again, write what happens here.

Keep cutting the halfs until you feel like you're completely satisfied with your story.

3. Detailing the story
Finally, you can use the timeline in order to put everything in two or three pages. Don't worry about the writing style. Just write short paragraphs in order to detailize the timeline. Don't write too much. Just what happens and why. Also, try to keep this on only two or three pages tops! Trust me it's much easier later.

4. Character Profiles
You can't really start writing character profiles if you didn't write the story. You may find yourself in a position where you realize that some characters are completely unnecessary or maybe some characters are missing. This is why it's best to make the story first and the break down the characters into bits.
It would be the easiest to write the personality of each character, physical aspects, abilities, weapons, height, weight and past/background. You can add more to this, but this should be something like the basics. It comes very in-handy later.

5. Strips
A strip is a drawing or sequence of drawings that tells a story. Written and drawn by a comics artist. Sometimes it is called "windows", as well. Before making strips, you should first write the scenes! This is also very important for orientating yourself when making manga.
Write how many strips you are using on each page and what happens in them, as well as who is saying what. This is helpful in case you make a mistake. It is much easier to erase something your wrote than something you drew, right?
Let's say that you want 4 strips on one page. Make one strip at a time! If you're using an image program like Photoshop or PaintShop, then first make one strip, then another, then the third, the forth and finally one blank page with dimensions of a comic boo (I'm sure you have a comic somewhere in your house. Measure it with a ruler perhaps).
Finally put them together.

I hope you didn't write anything in the talk bubbles yet. This comes last when all strips are put together!

And page by page you have yourself a chapter! ;)

Usually, publishers dont allow less than 10 and more than 30 pages per chapter so keep an eye on that too!