How do you get registered and start getting a real FIDE rating?

I live in Mexico, if it helps.

I can't speak for your area exactly but pretty much all you need to start is to play in FIDE rated tournaments. You don't register for FIDE, individual countries do.

For example, the USCF (United States Chess Federation) is a member of FIDE and USCF tourneys that are also FIDE rated (and run under the appropriate regulations) can qualify players for FIDE ratings. There are some other things that may limit getting a FIDE rating, such as playing enough opponents that have FIDE ratings and scoring well enough.

The approprate regulations can be found on the FIDE site:

For ratings:http://www.fide.com/fide/handbook.html?id=172&view=article

For getting a FIDE ID Number: http://www.fide.com/fide/handbook.html?id=12&view=category

Your national chess federation is FENAMAC (Federacion Nacional de Ajedrez de Mexico A.C.)

http://fenamac.org.mx/index/

I saw, seems like they don't have that many tournaments. 

There may be tournaments that are rated but which don't appear on the national site.