I am a us citizen, want to marry a french citizen who is here in the US on visa waiver program for 90 days how can I do that... Thanks Michael

Yes you can marry them, that does not mean they get to stay in the US, but you can marry them,

They would still have to apply to INS for a change in their status, may even have to return home to their county for a whie waiting on legal status, But you can marry.