Unless you're going to travel by car, the easiest and fastest way to get to Mexico and Central American is to fly.  

Many airlines fly to these countries -- here they are:  

Aeromexico www.aeromexico.com  
American Airlines www.aa.com  
USAirways www.usairways.com  
Continental Airlines www.continental.com  
Delta Air Lines www.delta.com  
United Air Kines www.united.com  
Mexicana www.mexicana.com  
Lasca www.centralamerica.com/  
Taca www.taca.com  
Northwest Airlines www.nwa.com  
Copa www.copaair.com  
Spirit Air www.spiritair.com 

You may also wish to try these popular travel sites:  
Travelocity www.travelocity.com  
Orbitz www.orbitz.com  
Priceline www.priceline.com  
Cheapfares www.cheapfares.com