Knowing a programming language is essential for applying to Research Experiences for Undergraduates (REUs), with popular choices including C++, Python, C, and Java. Many recommend starting with online resources rather than traditional books, as they provide ample information for learning any language. Skills acquired in one programming language are transferable to others. Additionally, studying numerical analysis is beneficial, as it aligns with common tasks in REUs, such as working with numerical data or maintaining equipment. Recommended resources for numerical analysis include "Numerical Recipes" in various programming languages. Legacy code in FORTRAN is still prevalent, and Python and Java are often used to support existing FORTRAN or C++ programs.