First time flying with stops?

So is Christmas season and I'm traveling from the US to Mexico for the holidays, I have already flying from point A to point B but I never have from point A to point B and C ( or city stops) so I'm nervous.
How early have to be on the airport?
Is one hour enough to switch from one plane to other?
How long can it take on immigration and still have to take the second plane?

They recommend arriving 3 hours prior to an international itinerary. You can sometimes get by with less, especially if you're leaving from a smaller airport and/or at a less busy time.

I try to allow at least an hour between connections. If I'm going through Immigration and Customs, then I try to allow at least 2 hours. How long that will all actually take depends on the specific airports you're using, and how busy it is on the day and time that you're flying. No one can really predict that for you.

You go through Immigration and Customs when you enter a country, not when you leave a country. Since you didn't list your airports, I can't really tell if you'll be doing that at your final destination or at your layover airport.

Where are you changing planes? If your stop is in Mexico, you will go through immigration and show your passport. If the stop is in the US, you'll just go to your next gate.
As for how early you have to be at the airport, that depends on which airport, what time your flight leaves, and whether you're connecting in Mexico or the US. You're supposed to be there earlier for international flights. If you're flying from one of the larger airports, it can take more time to check in, get through security and get to your gate. My airport doesn't open until 5 am, so even for a 6:00 flight, it doesn't do any good to get there earlier than 5.

Why do people always try to complicate questions. In general you want to be at the airport 3 hours before an International Flight. If you are traveling on a peak day perhaps add an extra hour if you are traveling from a major airport.

1 Hour transfer could be enough time, or it may not be enough. It all depends on if you are doing a Domestic or International Transfer, do you have to get your bags between the flights and are the flights in the same terminal?

Immigration could take 20 minutes or it could take 2 hours, again it all depends on the specifics.

If you want something more specific and actually useful then just tell us where you are flying out of, what airline(s) you are on, where you transfer planes, how much time you have and your destination.