Unlike “I” in English, there are a lot of way to say it in Japanese. The problem is that some word might not be appropriate to be used in some situation so it will help a lot to know the proper way to use each word.
Watashi – is the first pronoun you will be learning in any book and can be safely used in any kind of situations. It’s quite formal, but there is another word that have higher formality such as watakushi . There are also other alternate form such as atashi which have a more feminine nuance and therefore only used by girls, women or men who willingly use it to be feminine.
Note that unlike in English, Japanese don’t repeat the subject if it’s already clear. So most of the time, to look natural, you won’t need to use the “I” pronoun. The common mistake of beginner learners is to use watashi or other equal pronoun in every sentence they’re saying.
It might seems polite for foreigner, but some of Japanese might think of you as self-centered person. “I this, I that…, I already know who you’re talking about so why you keep on repeating yourself?”
Boku – commonly used by little boys with family members or friends. Have a bit of childish/pure nuance.
Ore – used by men and have a nuance where you put yourself higher than others. It’s considered rude to be used in normal situation. But it’s very normal for grown up boys / man to use it among friends. They seen it to have somewhat “manly/cool” nuance.
Hope it helps, happy learning!