Recent issue of Time magazine published an article describing the situation regarding ‘illegal’ or undocumented immigrants living in USA. According to that article there are more than 11.5 million undocumented immigrants, most of them are from Mexico.
However in that article they have included a raod map that you can follow to become an American citizen: path to get the Green Card which is an evidence of lawful permanent resident status in the United States, and eventually become American citizen. This has a flow chart which explains immigration options for different categories. There are different pathways you can apply, if you have a relative in USA, Born in USA, Degree or Special Skills or employer willing to sponsor you. It is easy to get a green card if you have a degree in a demanding field and a job offer; or if you are an extraordinarily accomplished or wealth investor.
There are other visa schemes also such as diversity visa programs and USA Green Card Lottery scheme.
So you Want to be an American: The road to become American citizen can go on months or even decades: