The short answer is that there is no right answer. The hardest word choice in any book is the handful of words that make up the title. And if I were allowed to give advice then the advice I would give is to trust in magical word angels who will eventually whisper in your ear and tell you what the title should be.
But let's say that the whispering word angels are on their coffee break and you want the long answer. There are some rules which, like all writing rules, have exceptions to when they can and should be broken.
The first thing to keep in mind is that most readers will decide to buy or pass on your book based on the title alone. Rightly or wrongly, that's what happens. That means that you need to put your marketing hat on when you write your title. You can put it on backwards like the cool kids do, but always remember that the entire reason your title exists is to sell your book. Think to yourself: Out of all of the books in a real or virtual bookstore what is the one title I can choose that will at least make someone stop and take a closer look? (Yes, yes, you're an artist not a businessperson. I know. Let's just say there's a reason the phrase starving artist exists, shall we?)
With that in mind, the first rule is that titles should be interesting, original, memorable and appropriate above everything else. If your book is funny, then the title should be funny. If your book is dark, then the title should be dark. If it's for kids, don't give it an adult title. If you're writing a romance novel, don't make it sound like it's science fiction.
The inspiration for your interesting and memorable title can come from several places:
People, places and things: Mystic River, Forrest Gump, Charlotte's Web
Expressions & Expressions with a twist: The Usual Suspects, You Only Live Twice, Of Mice and Men, The Grapes of Wrath, Live and Let Die
Thought provoking ideas from your book: Fight Club, Dances With Wolves, Catch-22, The Hunger Games, To Kill A Mockingbird, A Game of Thrones, The Green Mile, Rain Man
Another thing to keep in mind is that while titles are generally not copyrightable you can get in legal trouble if your title and subject matter are similar to another writer's (IANAL and all that). Beside legal trouble, if there are other works that share your title it can be confusing for potential readers to find your book. Lots of books with the same title can also make you and your book seem lazy and unoriginal.
The second part of your question is a little more difficult. If you're self publishing then there's nobody to blame for the quality of your title but yourself. But even if you go the route of traditional publishing you should still take time to find a title you love. Yes, if your publisher doesn't like your title they'll probably change it, and that's okay. But you should always, ALWAYS as a writer, care enough about your creation to fight to preserve the vision of your work. //Unless selling out makes you more money. Then you should totally cave on that whole "vision" thing.//
Some sources to scan for some more ideas: