Book titles are often duplicated quite by accident, and there is pretty much no way of preventing other people from publishing a book with the same title. It happens all the time, and as long as the title isn't something trademarked (like something in the Star Wars universe), it's generally not a problem.
I'd recommend you concentrate on writing the book, then when you have it ready for publication, search and see if the title is available. Don't get too hung up on the title - titles can change up to the last minute, and I understand it's not at all unusual for an editor or publisher to suggest a different title than the one you have in mind anyway.
You could register an ISBN, but that costs money, and again there's no good reason to do so before you absolutely need it. Especially since if you publish through a regular publisher, they'll take care of all of that anyway.