If you ever wanted to start running, you were probably encouraged to sign up to a 5k to keep focused on a goal. The same principle applies here: pick a CTF in the near future that you want to compete in and come up with a practice schedule. Here are some CTFs that we can recommend:
Wargames are similar to a CTF but are always ongoing. Typically, they are organized into levels that get progressively harder as you solve more of them. Wargames are an excellent way to practice for CTF! Here are some of our favorites:
There are some defense-only competitions that disguise themselves as CTF competitions, mainly the Collegiate Cyber Defense Challenge (CCDC) and its regional variations, and our opinion is that you should avoid them. They are unrealistic exercises in frustration and will teach you little about security or anything else. They are incredibly fun to play as a Red Team though!