If my mother was born in the U.S. and I legally entered the U.S. and have been living here for 15 years am I a citizen

As a starting point,
www.uscis.gov Forms N-600 Instructions

You may qualify to be a U.S. Citizen by a doctrine called jus sanguinis, (sp.) by blood, if your mother is a U.S. citizen, and meets the requirements for minimal years of residency (such as at least 5 years after the age of 14). You should read the instructions on the form N-600 for a precise listing of who may qualify as a citizen, even if you are over 18. If you ARE a citizen, the fact that you have been living in the U.S. would NOT become an immigration matter. The laws vary for those born when different laws were in effect.

Write down to collect Certified Copies of:
Mother's Birth Certificate
Parent's Marriage Certificate if any
Your Birth Certificate, Baptismal Records (if any)
Translation (if needed) of documents not in English
Certificate of Proficiency by the Translator
Any Registration of Birth of Child to U.S. citizen, available through U.S. Embassy or Consultates outside U.S.

You will need certain documents with your application if you are eligible to receive a Certificate of
Citizenship. This enables you to forego the immigration system, including naturalization, because of your
legal right by blood to citizenship.

Good Luck.

Whether you were born a US citizen or acquire US citizenship by some other operation of law cannot be determined with the information given.

Much more information would be needed to figure out your citizenship status.

That at least one of your parents was born is a starting requirement for you to have been born a US citizen. However, other conditions must have been met for her to have transmitted US citizenship to you at your birth.

The law as it existed at the time that you allege you acquired US citizenship determines whether you are a US citizen.

You should review your facts and options with an attorney.

Among other things, your attorney would want to know:
* your mother's history of living in the US before giving birth to you;
* your father's history;
* what was done at the time of your birth;
* what paperwork was filed for your entry into the US.

One of the first questions you'll need to answer is "how" did you legally enter the US? Definitely take any relevant immigration and documents related to your nationality to an experienced immigration attorney and review your history together!