I don't know about other self publishing companies, but CreateSpace creates an ISBN for you for free. They don't charge anything but their cut of sales, because they print on demand.
Definitely join writing groups, both online and in person. Get involved with other writers. Join authonomy.com. CreateSpace has a good system too, where you swap reads before publishing to get a feel for how others may view your book.
When I was writing my novel, I went over and over it many times. Then I had friends and family read it, and they found many, many typos that I had missed - things that spell checker wouldn't have found. Things like misplaced commas, apostrophes, and capitalizations. Also, factual matters like breeds of trees that didn't grow in certain areas. I had researched these things, but not in detail. All in all, it took longer to get the novel ready for indie-publishing than it did to write it! But it was well worth the effort.
Shannon