If you entered the country illegally over 20 years ago how can you become a legal citizen without marrying?

You can be adopted by a US citizen. You can be sponsored by an immediate relative that is a US citizen or a permanent green card holder. Marrying a U.S. citizen does not automatically confer citizenship or permanent resident status on the new spouse. Adoption is only allowed if the child is 16-years or younger has been certified an orphan by his or her country of origin and has been granted an IR4 visa for entrance into the United States. Contact the United States Citizenship and Immigration Services for information pertaining to the individual's specific circumstances or visit http://www.uscis.gov