Is there a need to cite a general idea in the introduction which shall be later developed with a reference?
Citations are only needed when you make a statement that requires external justification.

Thus, if you say: "This paper will examine X people in Y country" there is no need for external justification, since it is a statement about the paper itself.

When you say "X people in Y country are characterized by a habit of doing A, B and C", however, that's a statement that requires justification, either by external citation or by data presented in the paper.