Have a look at Royal Caribbean - Freedom, Liberty or Independence of the Seas to the Western Caribbean which includes a stop in Cozumel. If you want all Mexican resorts you would have to sail the Mexican Riviera from one of the California ports- Long Beach, San Diego- there are numerous cruiselines that sail out of there- Royal Caribbean, NCL, Carnival, Princess. Do you have a time of year in mind yet?

Depends if it is just a stop or a lot of Mexico,

on top of the suggestions so far

Panama canal repositioning cruises are worth a look.

Expect to pay around £1200pp(inside) for a 15-17n trip.

Mercury Panama Canal offer for £1149 today 16 night with the option to add on a stay at either end.
Need to look at TText as I am running out the door.

Also the other thing is to keep an open mine about where you are willing to go. As this is your first cruise then do look at other options as there are fantastic deals to be had out there.
Quality ships at rock bottom prices. I have never seen the likes before. I think that from now on until the end of the year we will see more of the same but if you see a cruise at a price that interests you then GRAB it or someone else will.

Welcome to the club and I am sure the bug will bite you but I promise just a teensy bit to start with.

Happy cruising!

Sue