Born in India. Parents not Indian. How to get Indian citizenship?

While we would need more details from you like your Date of Birth, your current Nationality, your parents Place of Birth & Nationality, your marital status and in case married, your Spouse's Nationality etc. to give you a more precise answer as we can apply these details to the  Citizenship Act 1955 and provide you with the best option to attain / acquire Indian Citizenship.

As per the Citizenship Act 1955, you can be a Citizen of India by Birth if:

a) you were born on or after the 26th day of January,1950, but before the 1st day of July, 1987;

b) on or after the 1st day of July, 1987, but before the commencement of the Citizenship (Amendment) Act, 2003 and either of your parents was a citizen of India at the time of your birth. 

Please also note that you would not be a citizen of India by virtue of this section (Citizenship by birth) if at the time of your birth-

(a) either your father or mother possessed such immunity from suits and legal process as is accorded to an envoy of a foreign sovereign power accredited to the President of India and he or she, as the case may be, is not a citizen of India;

(b) your father or mother is an enemy alien and the birth occurs in a place then under occupation by the enemy.

The other way for you would be Citizenship by registration wherein:

(1) Subject to the provisions of this section and such other conditions and restrictions as may be prescribed, the Central Government may, on an application made in this behalf, register as a citizen of India any person not being an illegal migrant who is not already such citizen by virtue of the Constitution or of any other provision of this Act if he belongs to any of the following categories, namely: 

(a) a person of Indian origin who are ordinarily resident in India for seven years before making an application for registration;

(b) a person of Indian origin who is ordinarily resident in any country or place outside undivided India;

(c) a person who is married to a citizen of India and is ordinarily resident in India for seven years before making an application for registration.

(d) minor children of persons who are citizens of India;

(e) a person of full age and capacity whose parents are registered as citizens of India under clause (a) of this sub-section or sub-section (1) of section 6;

(f) a person of full age and capacity who, or either of his parents, was earlier citizen of independent India, and has been residing in India for one year immediately before making an application for registration;

(g) a person of full age and capacity who has been registered as an overseas citizen of India for five years, and who has been residing in India for one year before making an application for registration.

Another option available as per the Citizenship Act is Citizenship by naturalization: 

(1) Where an application is made in the prescribed manner by any person of full age and capacity not being an illegal migrant for the grant of a certificate of naturalization to him, the Central Government may, if satisfied that the applicant is qualified for naturalization under the provisions of the Third Schedule, grant to him a certificate of naturalization:

Provided that, if in the opinion of the Central Government, the applicant is a person who has rendered distinguished service to the cause of science, philosophy, art, literature, world peace or human progress generally, it may waive all or any of the conditions specified in the Third Schedule.

(2) The person to whom a certificate of naturalization is granted under sub-section (1) shall, on taking the oath of allegiance in the form specified in the Second Schedule, be a citizen of India by naturalization as from the date on which that certificate is granted.

Hope this clarifies the options available to you.