I'm so glad you were able to get out of the shady publisher. WOW! I don't know a magic formula to make a book an instant best seller. If I did, I'd do it lol. I think it was important for me to figure out what my goal was, because honestly, no matter how successful you are, you are always going to want more. It's just natural to want to take the next step.
So don't get overwhelmed. You are going to be okay, and most important, this isn't a "one time chance" where if you make a mistake it's over. It really does work out.
I'm not sure where you are in the process. Has your book gone through beta readers and editing? Not everyone needs beta readers, but I found them super helpful. Are you planning to self-publish? Do you have a professional cover?
If those three things are a yes, then I would work on your blurb. You want to make it snappy without giving away spoilers. I hate blurb writing. Probably because I'm not super good at it, lol.
Then you need to decide if you want to go exclusively with Amazon to take advantage of their Select Program, or if you want to go "wide" and use other distributers.
Once you make that decision you need to decide if you want to do a paperback. I used Create Space. Don't get overwhelmed, just slowly tick this off your list. When you publish your 2nd this will seem much easier. It's just overwhelming because it's new.
Then you upload to however you want to do it, Amazon, or wide. I used Draft2Digital to go wide with my last book, and I am in select with my first 2. I wanted to be able to give my last book away for free, and this is the only way Amazon allows free, by price matching.
Make sure you preview your book before you publish it.
Other things before you publish... look for good key words (check out what genre you want your book to be in on Amazon. Play with key words in the search bar to see what books you can pull up. Amazon also has a place where it suggests key words to authors.)
Have a facebook landing page, and an email list in place before you publish so that you can put the links in the back of the book.
You can go to Goodreads and schedule a give-away to get more eyes on your book. I generally only give away 1-2 books and make my giveaway about a week long.
This is all just my opinion, and I hope it helps. There are all sorts of ways to publish books, and this is just one pathway. If there is anything else I can do, please let me know. Hang in there, and congratulations for finishing your book!