Following are some resources and books to get into competitive programming. Beyond this, it essentially just comes down to lot of practice, through practicing with past informatics olympiads and coding competitions on CodeForces, Codechef, Atcoder, etc.
USACO Guide
Competitive Programmer's Handbook - available here
Guide to Competitive Programming (Amazon link)
CSES problem set
Online Lectures on AlgoPlanet and SPOI
CP Algorithms website
Past ZCO problems on CodeChef
Past INOI problems on CodeChef