-- If you are asking, 'How do I show the character's thoughts in the manuscript?'
Narrative that talks about the character's thought is treated as any other narrative:
Susan was afraid of rats, having met many of the four legged variety in her rich and interesting travels.
Meredith pulled the hem of her dress from the widening pool of blood, thinking about forensic evidence.
If you wish to give us the exact words the character is thinking, you do this in Internal Monologue. This is represented in the book as Italics. It is cast in First Person, Present Tense. Thus:
I hate rats. I'm scared of them, too. Susan had met many of the four legged variety in her rich and interesting travels.
Meredith pulled the hem of her dress from the widening pool of blood. Let's not leave any forensic evidence.
Internal Monologue is represented in the manuscript in either Italics or with underlines.
No one who assesses the publication possibilities of your submitted manuscript will care which you choose.
I prefer underlines. They are easier to see. When the manuscript goes to publication, this pleases the copyeditor and the proofreader and everybody else involved.
-- If you are asking how much Internal Monologue you should add to the story and whether you should be dwelling upon thoughts rather than, say, action or dialog . . .
Write out the entire first draft. Think about the emotional impact you're getting with what you're doing. Look at the pacing. Find some good betas. Get opinions.
There is no general formula for how 'internal' a book should be.
There is only what works.
If you use, 'he thought', the first time you insert Internal Monologue, you will tell the reader that Italics=Internal Monolgue. After that, the reader will know and you do not need to say, 'he thought', again for Internal Monologue.
When you are dealing with narrative of the character's thinking process, you do not need to intervene with phrases like 'she thought'. 'she believed', 'she knew', 'it was obvious to her', 'she held the opinion that', 'it occurred to her'. You can just put in the thought. I mean -- it's obvious who is having the thought. It's obvious that it is a thought.
In the same way you don't have to say, 'she saw', 'she looked at', 'she noticed', 'she smelled', 'she felt'. Because you are in POV. it is crystal clear who is hearing or smelling or feeling.
She heard a bird begin to sing on the bush outside. Looking to the right, she saw Rory's eyelids twitch. She thought he'd be awakening soon. She knew she'd have to explain how they ended up in bed together. But in her heart of hearts she was aware there were no good explanations.
versus
[A bird began to sing on the bush outside. Rory's eyelids twitched. He'd be awakening soon. She'd have to explain how they ended up in bed together, wouldn't she? There were no good explanations.
Removing all that unnecessary direction to the reader puts us more deeply in the POV character's head.