The karate fist. Seiken
It is very important to have a corect fist when punching, many karateka spend to little time on developing a correct fist and they believe they have a strong fist, that is, until they hit something!
Once you feel comfortable with your fist, try hitting something, start with something soft, then build up to a heavy bag or makiwara (striking board).
Neglect this basic part of karate at your peril. A strong fist is essential. Ossu