First you need to have an idea. Sketch out some characters.
Unless you have a partner, you will need to know how to draw.
Although many mangaka decide the plot week-by-week, it is better to
have a good idea of the plot you want before you begin. Draw the
manga in pencil, then ink it. Drawing it is the easy part. Send
your manga to magazines and publishing companys. (Make sure to keep
the original and only send copies.) It is a good idea to send it to
companys that usually publish similar stories, but you should send
it to other types of places too. You might get lucky. Or you could
scan it and upload it on a website. If it gets popular online you
can self publish it, but it's unlikely you could charge as much as
it would cost to make each volume.
In Japan, there is the Comiket where fan-made or first time
manga authors sell their self-published / self-printed comics,
called 同人誌 doujinshi. In Malaysia, a similar convention is held in
December every year called Comic Fiesta, but on a much smaller
scale.
Shueisha has a monthly contest for new artists that you might be
interested in. See link below.
