Does an American travelling to Mexico need any special visas or travel documents other than a passport?

I’m going to be travelling to Mexico in about a month. I am crossing the border by land and driving to an airport in Mexico, then flying to another airport in Mexico, and travelling by bus to my final destination. I’ve been reading about FMM forms, but I don’t really understand what they are. Do I need to stop at an office as soon as I cross the border by car, or can I wait until I get to the airport for things like that? Are there any other documents I will need or things that I have to do? What about customs declarations?