When throwing a punch, the goal is to make contact with the first two knuckles (index and middle fingers). The knuckles of the ring and pinky fingers do not have as much skeletal support and are more likely to break. Many styles teach that the thumb should be tucked to the side, however Isshinryu karate teaches to put the thumb on top to strengthen the fist (as well as to provide another striking surface).
The fist should be almost vertical, with a slight turn to the inside. The punch described here fits very nicely into the space right below where the ribs divide. When punching high, angle the wrist down so you are still making contact with the first two knuckles.
A rule of thumb is "Out like an arrow, back like you touched fire." Basically, that means don't leave your punch out there - bring it back quickly so that:
1. You can throw another punch
2. You can block an opponent's punch
3. Your hand can't be grabbed (from which many bad things happen)THIS IS NOT ALL THERE IS TO A PUNCH!
Read this study, then go on to the next steps!
"The soviets looked at 120 boxers ranging from amateurs to experienced professionals. This study found that among the highest level boxers, the highest percent of their power (38.46%) came from the push-off of their back leg, whereas the arm extension and trunk rotation accounted for 24.12% and 37.42% respectively. "
I found this summary of the study on http://www.combatathletics.com/articles/Article_%20Punch_Power_Development.html
and several other web sites, but was unable to find the original study.