Which one is correct?
“why is he famous” OR “why he is famous”
If it is the former, why?
Another thing, which one is correct :
“another people” OR “other people”
Again, and why? 
Thanks beforehand
As a standalone question it should be “Why is he famous?” (Note the capital letter at the beginning of the sentence and the question mark at the end.) This is just the usual word inversion that occurs in English when forming a question:
“He is famous.” -> “Is he famous?”, “Why is he famous?”
“We were happy.” -> “Were we happy?”, “When were we happy?”
“They can speak French.” -> “Can they speak French?”, “How well can they speak French?”
etc.
The sequence of words “why he is famous” can occur as part of a longer sentence; e.g. “I don’t understand why he is famous”, “Why he is famous is a mystery to me”, etc.
“other people” is correct. “another”, since it literally means “an other”, cannot be used with plural nouns. (“another people” is possible in the less common sense of “people” as a singular noun meaning “ethnic group”.)
Wow Dozy, I can’t express how happy am I ! - I hope (am I) is correct - .
A very complete answer. Thanks a million.
In fact, it should be “I can’t express how happy I am”. This point is tricky for English learners to master.
“How happy am I?” (direct question)
“I can’t express how happy I am.” (not a direct question)
“Where are we?” (direct question)
“I don’t know where we are.” (not a direct question)
Sometimes you can get questions without the inversion, but the uninverted words are never direct questions, they are part of a more elaborate structure. For example:
“Do you know how happy I am?”
(In this case the actual question-forming inversion is in “Do you”.)
i guess what everone said lol. they beat me to it.
Another thanks Dozy.