How can you prepare for competitive programming contests?

Here's a curated list of resources that a lot of people refer to: https://github.com/lnishan/awesome-competitive-programming

But really, the way to get started is just to pick a site and start solving problems. If you're specifically interested in GCJ then it's best to practice using previous problems from that contest. But I wouldn't recommend getting too caught up in one contest. If you start with the Easy and Medium problems on HackerRank or LeetCode, there are tutorials, discussions, and editorials to help when you get stuck. And those sites are easy to use, so you can get a lot of practice time in with low friction. That will get you past the beginner stage.