The 2 big organizations are North American Strongman and United States Strongman. If you google those names, you'll find their websites (can't post links here). Also, Kalle Beck runs a great website called Starting Strongman, which has a competition finder. He also has a facebook group you can join (same name) that will be helpful.
Don't wait too long to compete. There are novice divisions in most contests that are great for someone new to the sport. You will get WAY stronger competing than you will getting ready to compete.