Believable or Not?

A good author wants the plot to be believable.  Characters should be believable.  The problem faced should also be believable.  Popular books and great literature normally follows this pattern.  Forbidden love forces young lovers to hide their relationship from their family - Romeo and Juliet.  An evil King, or a crooked politician is the story as old as time - inspiring great literature from the Three Musketeers all the way to Hemingway and Steinbeck.  These are believable characters following the problems of a believable situation.  So how do we create this for our novel?
When people read novels they want the fiction to be so close to reality that it deceives them into believing this could actually happen.  Of course, this is not always the case.  Much science fiction is simply ridiculous and could never happen - but the stories, if told well, still make this seem possible.  The age of online reading has made this possibility seem even more real, since we can add to popular books the endless barrage of fan fiction and analysis.  Since the advent of the internet, online reading has made it possible for anyone with a keyboard to become a contributor to online novels.
So do you want your novel to have a believable plot?  If you are writing a children's book the answer may be "no". You could write a long novel about a caterpillar who wants to be an elephant.  For ridiculous fiction the reality is not important.
But imagine you want to write a great romance novel.  You have to begin with characters that seems real.  They have to have believable lives and problems and the story cannot take place in a fairy land with bean stalks and giants.  Our main character needs flaws and weaknesses - like every human you have ever met.  Creating a perfect character does not mean the character will be perfect.
Of course if this is a romance novel about fairy's and giants then you can disregard the last part - but we are discussing here the illusion of reality in our fictional novel.  When someone reads our novel they want to be whisked away to a believable place that might exist; because if it does exist then that means there is the slim chance that they could live this life also.  They can find true love, or experience a real adventure.  That is on your shoulders as the author, make the story seem real.
I believe you ca do it, if millions of crazy Star Wars fans can believe the reality of something like that, then certainly you can make a story seem real if you write about what you know.  Don't include details of things that are beyond your grasp.  If you have never been to a beach, don't set your novel on the beach.  If you don't understand the laws of thermodynamics, then don't introduce any rocket scientists into your novel.  It's not impossible, with research, to include things you don't understand; but why not stick with what you know for now.  If you are a waitress, write about a waitress.  It will be real, it will be believable, and the literature world will thank you.