Interested in an advanced degree in Economics? Here is some helpful advice for getting into a PhD program:
1. MATH!!! This is the biggest surprise for juniors and seniors who decide to look into grad school only to find that economics PhD programs require a lot more math than OU's economics major. You should take as much math as possible; it's not an exaggeration to say that a math major/econ minor is a more attractive applicant than an econ major/math minor.
2. As far as economics classes go, you should definitely take Principles and Intermediate for Macro and Micro. After that, you should take statistics (ECON 2843) and econometrics (ECON 4223); these courses are sufficient for a minor in economics.
3. Most graduate schools will require 2-3 letters of recommendation from economics professors. Letters from professors in other fields, employers, or anyone else might be relevant as additional recommendations, but the main ones should be from econ faculty.
4. You'll need to take the GRE; your quantitative reasoning score matters much more than your verbal reasoning score, but very competitive schools will look at your whole score.
5. More math. Seriously. As much as possible. Ideally you'll have 12 credit hours of calculus along with linear algebra (MATH 3333), differential equations (MATH 3113), discrete mathematical structures (MATH 2513), and intro to analysis I and II (MATH 4433/4443); incidentally, if you have all of these, you'll have a minor in math at OU.
This is only a basic list. Things like your personal statement, research experience, TA experience, and selecting a graduate program that's right for you are also important factors in the application process. It's a good idea to talk to a variety of people and look up information from a variety of online sources (especially people who've recently gone through the process).
Graduate schools vary quite a bit when it comes to admission standards and the types of programs they offer. It's never too early to start researching your choices!