@Ross at PlayRoss's summation is excellent. Internal thoughts, when used, can be marked either with single quotes (ex: 'I can do this,' he thought) or with italics (ex: This is the perfect opportunity for me to impress her, he reflected.) But as Ross noted, they key is being consistent, not just throughout your story, but also across any further stories you write. If you decide you made a mistake you can change it, but it's best if you go back and correct the alternate uses in your already posted stories, just so your regular readers know what to expect when reading your stories.
@oldegrump
Perhaps is you are using mind reading, you would use another voice, but to me if you show distinct separation it is not needed.
What oldegrump is referring to, is a convention that many authors have chosen when invoking telepathy (the ability to communicate mind-to-mind with others) so their dialogue is conveyed as the other characters 'thoughts', but which is distinguished so readers know it's not their own internal thoughts. This convention is to combine to two alternative formatting techniques (ex: 'You are now my slave!' Tom projected into Betty's mind. 'The hell I am, motherfucker, she responded, telekinetically kicking him in his nuts.)
But, you'd only need such formatting in science-fiction, mind-control or superhero stories. Otherwise, I'd avoid it altogether.
Expansion on the ideas:
The reason why you don't use internal thoughts in 3rd person stories (a story from someone else's perspective), or for anyone other than the main character in 1st person stories, is because there is no way for the characters (which includes the narrator) to know the internal thoughts of other people.
This practice is known as "head hopping", where an author projects their own thoughts (the author's thoughts) into the mind of other characters. It's frowned upon because it typically takes the reader out of the story and into the author's thoughts about the character's motivations. Instead, it's best to show how a character is thinking by how they respond to events (i.e. revealing their motivations by their physical action, rather than TELLING the readers what he internal thoughts are).
If you Google "head hopping", it'll give me more details on why you don't want to venture into this territory, although many, many authors do it anyway.