How can an illegal citizen who has been here in the US for over 5 years become a legal citizen of the US?

Well the most common way to do it is by marriage as you just mentioned it yet i don't know yet the procedures of that situation however if you are an illegal immigrant you cant become a US citizen you will have to start with permanent residency and than maybe you can become a US citizen but try ghetting married ina lawful way dont pay someone to get married with you or else you will be in a whole lof trouble.