While its a great thing that you wrote all this, I doubt that anyone on these forums knows what would be the ideal step for you.
If you were to write a book your situation would be a bit easier, I assume, however, you want it to be made into anime...
Thing is (although Im not sure) that the Japanese anime industry is probably not that open to "external injections". I mean, not to sound too harsh, but you're not the one and only person with a great idea. Studios probably get loads of mails and whatnot saying that "I have teh story for you!", and they would gladly make those wonderful plans into reality, but there's one problem... budget.
If you want to reach your goal, you'll probably have to spend a couple of years on short term stuff... I mean you need reputation in Japan, let us see the steps leading to that.
1, Being in Japan:
a, You have to speak the language perfectly.
b, You need a Visa, a long term one preferably.
--- You might need a degree for this.
c, You need money, plenty of money.
d, You need even more money since you probably won't work right after arriving.
e, You need a job, something related to the anime industry.
--- You need a degree for this. Scriptwriting, multimedia or something among these lines.
f, You need a place to live.
Now lets say you accomplished all this.
You have to be successful in your work:
a, You need patience and time, not many get famous in their first year of work.
b, Luck, yes you need a bit of luck too.
c, You need to convince those people that they can trust your plan.
b, You need to satisfy the audience. Something that is great, has awesome depth, is scary, transcendent, etc. is probably not edible to the masses raised on shounen filth.
So you'll have the face the fact that they might cut down from or add to your plot.
And last but not least, you must be aware that not being Japanese (I assume) might hinder the process of achieving whats listed above.
Anyhow, I wish you the best. I myself am preparing to be a scriptwriter, so I thought about this issue a lot.
One thing that I must add to my post:
Don't give up. What you have in mind is not as grandiose at the beginning, but surely, it doesn't lack grandiosity once you have achieved it.
This task is not difficult because its actually hard, its difficult because not many choose to complete it.