A citizen is a person with full rights in a country. A person can become a citizen of a country by birth, registration or naturalization.
Citizenship by birth is acquired when a person is born in a particular country however, not all countries recognize the birthright citizenship. In Nigeria, a person is a citizen by birth if either of his parents or grandparents belongs or belonged to a community in Nigeria or if either of his parents or grandparents is a citizen of Nigeria and every person born outside Nigeria, either of whose parents or grandparents is a citizen of Nigeria. See section 25 of the 1999 CFRN (as amended)
Citizenship can also be acquired by registration. In Nigeria, a person may acquire citizenship by registration if the president is satisfied that;
a. he is a person of good character
b. he has shown a clear intention of his desire to be domiciled in Nigeria
c. he has taken the Oath of Allegiance prescribed in the Seventh Schedule to the 1999 Constitution.
A person who wishes to naturalize may apply for the grant of citizenship by naturalization. In Nigeria, the president may grant a person the certificate of naturalization if he’s satisfied that;
a. he is a person of full age and capacity
b. he is a person of good character
c. he has shown a clear intention of his desire to be domiciled in Nigeria
d. he is a person who is capable of making useful contribution to the progress and well-being of Nigeria
e. he has taken the Oath of Allegiance prescribed in the Seventh Schedule to the 1999 Constitution. Etc. See section 27 of the 1999 constitution (as amended)
https://lawstudentaide.com/2019/01/17/citizenship/