Best advice I've seen thus far for someone outside of japan to publish a manga has come from mark crilley. He does tutorial/advice videos on youtube about manga, and how he started doing manga.
1: For starters, don't expect it to happen anytime soon. I'm not talking for one or two years. Realistically, it probably won't happen, and you have to be prepared for that. Even if it does happen, it more then likely won't happen for at least 5-10 years (Unless you get insanely lucky). It takes years of trial and error before most people even get their stuff looked at, never mind published.
2: Work hard, and don't stop. Given how saturated the art field is with exceptional talent (especially the manga/comic scene over the past ten years), you have to prove that you're ready to work your fingers to the bone to reach your goal. No one is going to want to publish a series done by someone who's heart isn't in it.
2.5: (Tied into #2) Never stop trying new things, and studying different forms of art. This will ensure that your art work has a different feel to it then others, and thus, will stand out from the pack so to speak.
3: When you're finally ready to look for a publisher for your work, go to smaller publishers and don't even bother with the big ones. Big publishers like tokyopop, viz, etc. get literally hundreds of entries a month looking to get published. If you go to a smaller publisher that only gets 2-3 manga/comic proposals a month, which one do you think is going to give you better odds of getting published? (If you just thought "well mine is better then anything else out there, and will get picked up my a big company as soon as I send it in" You're wrong. It may sound harsh, but its true. In the art world, there will always be 50,000 people who are considered "better" than you. So it's your job to be more unique, bring something to the table they haven't seen, and regardless of whether someone else is "better". They'll pick you over them every time.)
4: Set realistic goals. Don't set your self up by saying "I'm going to be the next naruto, death note, etc. Within a year". Because that will just end in tears. A more realistic goal is something like "I'll get a one shot 30 page manga/comic published within the next 5 years". Once you've done that you can start making your goals more and more ambitious.
5: Make sure you're ready for what will be required of you. As most professional manga/comic artists will tell you, with the ability to get your work published, comes a loss of sleep, free time, and money. Going from "amateur" to "professional" is where the hobby artists are separated from the serious ones. As the hobby artists will back out the second their fun loving hobby becomes their job, and begins to suck up all their time and offers very little money for compensation. Professionals however, will keep doing it in spite of this as this is what they love to do, and regardless of the hardships. Don't want to be doing anything but art as their 9 to 5. That being said. It's common misconception that a "professional" artist is naturally better then a "hobby artists". The only difference between the two is a pay check. They both love what they do. One just does it as their job, while the other would rather not have what they love to do, become work. This by no means, means that one is more skilled then the other by title alone.
6: Make sure you consider everything seriously, and decide which group you fall into. Hobby artist, or aspiring professional. If its the latter, then the advice is there. Just make sure never to forget why you wanted to become a professional in the first place. Everyone questions their choices from time to time. So long as you never loose sight of why you're doing it, you'll never be at a loss for inspiration to carry on when you fall on hard times. If you fall into the former category, then just carry on enjoying art for what it is. Fun. And don't worry about getting published, or taking it overly serious.
Mark Crilley's youtube page> http://www.youtube.com/user/markcrilley