If you are considering applying to a PhD program in Economics (or some related areas), here are some useful materials:
Should I get an Economics PhD?
Some people consider a PhD in Economics more useful than other kinds of PhDs. See why in: If you get a PhD, get an economics PhD. Nohapinion.
Here is a brief description of the graduate economics degrees by the American Economics Association.
For a general overview of why consider studying a PhD I recommend this article: FAQs on PhD applications by Chris Blattman.
Here is a thought about whether to get a Phd or not.
Some general considerations for prospective graduate students in economics and useful links by the AEA are available here.
If you're not sure about a PhD. Maybe consider a Master's Program instead? How to choose a Master's program? (mostly focused in Public Policy): Which Master’s program is for you? by Chris Blattman.
In Spanish: ¿Cómo Decidir? by Aplicar desde México.
What should I have done before applying to a PhD program?
For a general idea of the profile that top universities are looking for in their applications, check here or here.
As general advice for aspiring economists (regardless if you want to do a PhD in economics) see here.
Math classes: Besides the general courses (Calculus, Linear Algebra, Probability Theory, etc.) I strongly recommend taking a Real Analysis course (sometimes elective as an undergrad).
For a review of the math topics needed in the PhD, check the Math Camp course of the University of Arizona.
How should I prepare the application?
Once you have decided to apply for a PhD, here and here are some recommendations for the overall process.
For a detailed guide of the process, see: Applying to PhD Programs in Economics: An Extensive Guide.
Choose where to apply: Here are some recommendations.
Letters of Recommendation: What do letters do? Who should you ask? See: Everything you need to know about letters of recommendation by Chris Blattman.
Statement of Purpose: How to write it? Some advice? See: PhD applicants: Writing your statement of purpose by Chris Blattman.
TOEFL preparation: It's useful to have a preview of the type of questions and the structure of the desirable answers before taking the exam. One good set of classes for people with a good level of English is: TOEFL Classes, Greg Mat.
GRE: Mostly focus on Math, verbal is sometimes not even considered in the requirements, but check every university webpage. I recommend first reviewing the topics and then practicing to reduce the time spent in solving each question and getting used to the test environment.
For the review, many people recommend using some books for quantitative reasoning like this or this (check them out in the used section of Amazon or in a library) and the Magoosh platform (has very good videos).
For practicing, the magoosh practice tests and a book of exercises like this or this could be useful. Don't forget to use your exam simulations included when you purchase the exam (I recommend doing them closer to the day that you will present the exam).
For vocabulary, I suggest the Magoosh's free GRE vocabulary flashcards.
After applying, where should I study my PhD?
Once you have applied and received some offers you need to consider a great amount of factors before making the final decision on where to study your PhD. This article summarizes them perfectly: Choosing a PhD program: Advice from recent PhDs. J-PAL.
Other resources:
General page of resources of the American Economic Association: Preparing for graduate school, AEA.
Recursos para estudiantes latinoamericanos acerca del PhD, la Academia y mucho más: Ver la página de EconThaki.
En español, recomiedo mucho ver el enlace de Cómo aplicar? y toda la página de Aplicar Desde México y Aplicar desde Argentina.