U.S. citizenship is not conveyed through marriage, so your marriage to your girlfriend would not make her a citizen. In fact, it would not automatically convey any immigration status at all.
Whether your girlfriend would be eligible to receive lawful permanent resident status based on your marriage depends on many circumstances, including whether she has departed the U.S. and returned unlawfully at any time since her original entry, whether she has adverse immigration or criminal history, whether the two of you can prove that you married because you intended to establish a life together (and not just for immigration reasons), etc.
The process involved is going to depend on how she entered the United States (with or without lawful documentation) and whether anyone filed certain immigration paperwork for her or for her immediate family members by April 30, 2001 or prior. It would be best to consult a licensed, experienced immigration attorney for verification of eligibility and the process involved in obtaining residency based on marriage.
For assistance locating immigration attorneys or non-profit organizations in your area, you can look here on Avvo, at www.aila.org, www.ImmigrationLawHelp.org, or http://www.justice.gov/eoir/probono/states.htm.
By the way, she currently can attend public colleges in California at in-state rates, even without lawful immigration status. That law is called AB540 if you want to look up more information about it.
Ms. Doerrie's answer to your question is general in nature, as not all facts and circumstances relating to the specific person(s) and situations involved are known to her. Ms. Doerrie recommends consulting with an immigration attorney regarding your specific facts and circumstances prior to making any legal decision or submitting any form or application. This response does not create an attorney/client relationship.
No lawyer will recommend getting married for the purpose of giving anyone an immigration benefit. Having said that, if you do marry her and she is less than 180 days past her 18th birthday then she should leave right away. Any unlawful presence in the US before the age of 18 and up to 180 days after her 18th birthday will not trigger the mandatory bar on her re-entry to the US. Then you simply hire a lawyer who can file for her immigration and prepare a brief to the USCIS in such a way that it does not cause her any issues. Then she would enter the US with a green card and three years later she could apply for citizenship.
Talk to a lawyer ASAP!!!
Khaja M. Din, Esq.
Free Consultation For Your Immigration & Deportation Issues
I agree with Atty Doerrie.
(213) 394-4554 x0 Mr. Shusterman is a former INS Trial Attorney (1976-82) with over 35 years of immigration experience. His response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.