A couple of smaller publishers might be open to submissions that involve a partnership between an author and an illustrator, but most publishers only want to see the text, no illustrations, and you should include art notes only if they're necessary for the story. (If you're both an author and an illustrator, you can submit both, but this doesn't apply in your case since someone else did the art.)
Some publishers accept submissions directly from authors, but many only accept submissions from agents, so if you want to reach them, you'll have to query agents first. Most people recommend having at least three polished manuscripts before you start querying agents. You don't need an ISBN at this stage, and you don't need to register the copyright before you submit, either.
If you decide to self-publish, everything is different. You need to supply the art, so you can use the art you have, and you'll need to register the copyright and get the ISBN, along with formatting, distribution, marketing and everything else. I'm not completely sure on the order, but the next step would probably be to decide which self-publishing platform you're going to use (IngramSpark, KDP, etc.) and see what their process is.
If you're hoping to sell the manuscript to a traditional publisher, this probably isn't what you want to hear. You might consider separating the text from the art and submitting the text only. (As long as you have the full rights to do so. I'm not sure what kind of arrangement you have with the illustrator.) Alternatively, you might try to look for a small publisher who accepts author/illustrator partnerships -- it's far from the norm but I've heard of it once or twice.
There's a lot to learn when you're starting out, but you've come to the right place. Good luck!