I say start typing something up, or writing then typing if you prefer. Once you have something to work off of and want it critiqued I'd be happy to help with grammar and vocabulary. Wouldn't dream of touching the story, that's YOUR domain. But if you really want this written out: I'd suggest getting to it RIGHT AWAY. That means: Don't let anyone discourage you before you've gotten out at least a couple chapters. Feel free to think it is great, but expect to go over it again later, and brutally. Then when you give it out for critiquing, be ready to accept views but take only what helps your story. Take time to see if what they say makes sense, objectively. This can be hard, I know, especially if you take great pride in your work but if you are willing to learn your stories will truly begin to flourish.
Now, take it from one 'author' to another, you want to get those ideas onto paper while you still have the urge. The more time you let pass, the more the idea will begin to bitter. Just like tea, the conception stage must be: 'just so' in length, anything more ruins the brew.
Then I'd suggest getting out at least a few chapters before bothering to review it, now it probably won't be 'great' but at least you'll have it out. Then once this is done if you need encouragement and assistance my door is always open. Just send a shout and I would be glad to assist and encourage. Mistakes are just methods of learning what not to do after-all and not all points of edit are 'mistakes'. Some just require style development to implement.
So if you need a firm but soft hand to assist, I am here. Get something out first, though.