Book
Competitive Programmer's Handbook
Free, comprehensive, and the most commonly recommended starting text.
Book
Introduction to Algorithms (CLRS)
The standard reference for algorithm theory and analysis.
Platform
Codeforces
Regular contests and a huge archive of rated problems.
Platform
CSES Problem Set
Structured practice problems organized by algorithm topic.
Platform
AtCoder
Well-structured contests, especially strong for beginners.
Tool
VS Code
The editor most coaches recommend for contest practice.
Tool
Compiler Explorer
See exactly what your code compiles to — useful for optimization.
Book
Elements of Programming Interviews
Problem-style practice with detailed, worked solutions.