You have two options, if this is a book you don't intend to publish.

The low budget option is to google resources to learn commas and then attempt to execute it, maybe getting a friend or a friend of a friend with an English degree to look over it when it's done.

The other option is to evaluate how much your grammatical errors bother you in actual dollars. A 50,000 word book will run you $100 to $200 for a real editor online with a Master's in English to do a complete run down of every error and send it back to you. Maybe it's worth it to you, if you intend to share it with a lot of people, or it's just important to you to have a copy that's perfect for yourself after working so hard on it.