Be fearless but don't be stupid
Punches fail because of fear, not because of lack of strength. The less your brain perceives the board as a barrier, the more likely the punch will succeed. For beginners, aim beyond the board, i.e., follow through! As any martial artist can attest, the hand hurts more when you fail to break than when you succeed. Being fearless does not mean you should be stupid. Do not attempt breaks without considering the material, the state of the material, and you ability.
Keep hand in a tight fist
Your punching arm should be tight, but all other body muscles should be relaxed. Some martial arts require the thumb of the fist to point upward in a punch. Taekwondo requires that the thumb face inward. Biologically speaking, the strongest support for the punching hand is when thumb is not up or inside but relaxed so it naturally points 45 degrees inside. Punch contact area should be first knuckles of index and middle fingers. This offers minimum contact area for a greater breaking force and it keeps the contact area in line with the forearm to reduce chances of spraining the wrist if the punch is not successful. Punching with other knuckles forces wrist to bend and may result in a sprained or broken wrist. Always punch in middle of board.