I want to learn grammar ASAP. when should I start ideally?

Once you start learning grammar, you’ll want to find a way to be exposed to it, such as through reading or listening to native material. But it can be difficult when the native material is 98% grammar you haven’t learned yet.

One option is to join the Absolute Beginner Book Club, either by reading the current book, waiting to join the next one, or picking a previous book to read through. You get the advantage of vocabulary lists and discussion, the latter often covering various grammar. Plus you can still ask questions on the book club threads for previously read books/manga.