I want to know exactly what happens if I marry an Illegal Immigrant. Does he become a temporary resident and then we file papers for him to be a permanent resident ? How much does it cost and how long? What do we have to do from the very beginning of getting married? What papers does he have to have?

Marrying an illegal doesn't give them a benefit to becoming a citizen any longer.

I could be wrong, but I've heard it takes up to 7 years for someone to become a legal citizen.

When you marry a illegal person in the US, the only thing that happens is that you are married, they are still illegal. And if caught can still be deported, so you want to move to their country if they get deported?

They may still be required to move back to request citizenship and even if not, the process can be very long.

You need to involve a immigration attorney to start the paper work going

I want to know exactly what happens if I marry an Illegal Immigrant. Does he become a temporary resident and then we file papers for him to be a permanent resident ? How much does it cost and how long? What do we have to do from the very beginning of getting married? What papers does he have to have?

If he entered US with a visa but overstayed, you should have an immigration attorney handle the paperwork. He doesn't need to leave US. It takes between 8-10 months to become a permanent resident with 2 years condition.

If he entered US without inspection, he must return to his home country while you proceed immigrant visa for him in US. He can't wait in US. It could take up to 12 months or longer depending on what country he's from. You need an experienced immigration attorney.

Application fees are approx $2K, attorney fees vary.

And if he has to apply for the IV in his home country, prepare for a waiver application depending on the nature of the US immigration violations.