One of my first pieces of advice to new writers is: When you've got something in your head, bursting to come out, just write it down.
You have an idea for a book you want to write. I'm assuming you want to publish it for an English-speaking audience.
Start putting the ideas on paper or your computer or a notebook, or whatever. Don't worry about the perfection of your English.
At some point, find someone (hopefully in the area where you live so you can meet with the person) and ask the person to look over what you wrote. As an English instructor, I can say it is easier to correct the grammar and sentence structure where the content already has value and substance, than to try to add substance where the English grammar is perfect.
Let me toss some questions to you as you start writing: What is your main idea? Who is the audience you want to reach? How do you want the audience to react to what you have written?
If you're clear on those goals, that will help you as you put words into sentences, sentences into paragraphs, paragraphs into chapters.
Don't worry about perfection at this point. Just start writing. You will find that as you write something down, other thoughts will come. And you may find that the actual "act" of writing will help you in improving your English.
At any rate, go for it! Best wishes.