In a plain positive sentence, the some- words usually feel right: something, somebody / someone, somewhere.
something, somebody, somewhere
I have something to tell you.
Somebody called you this morning.
Let's go somewhere quiet.
Not "I have anything to tell you." — say "I have something to tell you.". A plain positive sentence takes a some- word, not an any- word.
Try it: There is ___ at the door. I can hear them. (somebody / anybody / nobody) Answer: somebody — This is a plain positive sentence (there really is a person), so it takes somebody. "Anybody" is for negatives and questions, and "nobody" would mean no one is there.
anything, anybody, anywhere
When the sentence is a no (don't, isn't) or a question, the some- word usually turns to an any- word: anything, anybody / anyone, anywhere.
I don't have anything to say.
Is there anybody home?
We didn't go anywhere yesterday.
Not "I don't have something to say." — say "I don't have anything to say.". "Don't have" makes it a no, so something turns to anything.
Try it: Did you see ___ at the station? (anyone / someone / no one) Answer: anyone — This is a question, so it takes anyone: Did you see anyone? "Someone" is for plain positive sentences, and "no one" would already mean you saw nobody.
nothing, nobody, nowhere
The no- words already mean not by themselves: nothing, nobody / no one, nowhere. So the verb stays positive with them. Do not add another not.
Nobody came to the meeting.
Nothing happened.
There is nowhere to sit.
Not "I don't have nothing." — say "I have nothing. / I don't have anything.". nothing already means not, so do not also say don't. Pick one.
Try it: The fridge is empty. There is ___ to eat. (nothing / anything / something) Answer: nothing — The verb "is" is positive, and you mean not anything, so use nothing: There is nothing to eat. "Anything" would need a no first (isn't anything), and "something" means there is food.
Try it: Which one is right? (We saw nobody. / We didn't see nobody. / We don't saw nobody.) Answer: We saw nobody. — Nobody already means not one person, so the verb stays positive: We saw nobody. "Didn't see nobody" puts not in twice. (You could also say "We didn't see anybody.")
everything, everybody, everywhere
The every- words mean all of them: everything = all the things, everybody / everyone = all the people, everywhere = in all places / every place. Even though they mean many, the verb stays singular (like is / was / has).
Everybody is here now.
She told me everything.
I looked everywhere for my keys.
Not "Everybody are happy." — say "Everybody is happy.". Everybody means all the people, but it takes a singular verb: is, not are.
Try it: The party was great. ___ had a good time. (Everybody / Anybody / Nobody) Answer: Everybody — You mean all the people had a good time, so use Everybody. "Anybody" is for negatives and questions, and "nobody" would mean no one enjoyed it.
Try it: I looked everywhere, but I couldn't find ___, so in the end I bought ___ new. (anything … something / something … anything / nothing … anything) Answer: anything … something — "Couldn't find" is a no → anything. "I bought" is a plain positive → something. A no takes any-, a positive takes some-.