Basically the rule for writing a novel straight up is to not change your POV.

Here is an article about POV

Usually for fanfics that isn't followed but bent really really bent. Multiple POVs are, in my opinion, an illusion. Why?

Even if you have a story where you SHIFT the POV, the kind is the same. It's first person POV that shifts from one character to the other. 

You are still supposed to keep the POV the same unless you want to shift it. Okay. That's confusing right?

BASIC RULE: Don't shift from Third Person POV to First Person. 

Why? It's hella confusing for your reader. Grammar Nazis will stop reading your story after catching this error. And they are unlikely to come back again. 

Also it makes it easier to know who the heck is talking in the story. And the pronouns won't hella confuse you. 

Here is a simpler tip list about POVs check it out.

How do I not switch incorrectly?

Re-read, revise and line it up. Make sure that you read your work after you have written them and keep it on draft until you fixed it. Read the chapter before to make sure that you didn't shift your POV. 

Why use the first person POV? 

According to the style or theme of your story, it would make things more intimate. If the story is about an introvert, then first person POV works better because an introvert doesn't usually speak their mind. So in order for the inner face (internal personality) of your c