Many writers struggle with point of view (POV). I've struggled with it myself and still find places where I've slipped into a different POV than I should be in. What's the big deal with POV, you ask? Let me see if I can explain it in a satisfactory manner.
Point of view is the perspective from which you write. The POV character is the character whose eyes the story is filtered through. Here's an example. Let's say you have three characters: Bob, Anne, and Sally. Anne is your POV character. Everything in that scene has to be observable by Anne, which means her thoughts are the only ones you can include. Now, through Anne's observations, the reader can get a pretty good idea of what Bob and Sally are thinking, but showing Bob's and Sally's thoughts is a big no-no when writing in Anne's POV.
Clear as mud, huh? Let me continue.
If you have the scene start in Anne's perspective, then you switch to Bob, then Sally and back, that's head-hopping. It can be jarring, distracting, and just plain annoying for the reader. I know it's tempting to show all the characters' thoughts and feelings, but your story is much better if you stick to just one POV per scene. If it's a long scene and you can make a smooth transition, it's okay to start in Anne's perspective and subtly shift to end it with, say, Bob's. But never ever skip back and forth between points of view. It's also good to limit the number of POV characters in your story. Some people recommend using only one POV throughout an entire book; others say two or three points of view are fine as long as they're in separate scenes. A lot of romance books have two perspectives: the hero's and the heroine's.
To figure out which character or characters should be POV characters is pretty simple. Who's your main character? That should definitely be a POV character. If you have two main characters, you should probably have two POV characters. If you have six or seven main characters, you might want to reexamine the story. Too many main characters can get confusing to anyone trying to read the story. A lot of times, you won't have more than two main characters. You may have very important secondary characters, but they're the supporting cast. The main characters are the lead roles, like in a play or a movie.
"How do I know who's POV I'm in?" you may ask. That's real simple. Put yourself in the POV character's shoes. If you were in that situation in real life, could you see, hear, touch, smell, or taste whatever it is? If the answer is yes, you're in the correct POV. If the answer is no, rewrite the bit until the answer is yes to get yourself back in the correct POV. If you have a hesitant maybe or kind of, you should probably rewrite it just in case. You never know when you're going to run into an editor who despises borderline POV slips as much as head-hopping.
One thing I've learned over the years is: when in doubt, rewrite. Just when I think I can slip something past a reader or critiquing buddy, they'll catch it and say, "Wait a minute. Are you sure about that?" No, I'm not sure, but I was hoping no one would notice. I've learned to research until my brain is so full of information it's going to explode (think of cramming for a final), then writing according to fact so that even when it seems wild and out there I can say, "Yes, I researched that and it says right here that's the way it is."
The same thing applies to POV. I've tried to sneak minor POV slips and borderline POV slips past critiquers only to have the errors pointed out to me. When I revised to fix the POV problem, I ended up with a better story.
There are a lot of resources online and in libraries and bookstores that can you help you understand POV. Critique groups are another great resource. Chances are at least one member knows about POV and can guide you to a deeper understanding of the sometimes tricky, often exasperating subject.
Happy writing! (Just make sure to stick with one POV per scene.)