Lose Your Cool
A car crash is never a pleasant experience. Afterward, your emotions are usually running high and you may even be injured. In spite of this, it's never a good idea to lose your cool, especially at the other driver. (Yes, even if the wreck was their fault.)
When you're dealing with the other driver, the first thing to ask is, "Are you alright?" Don't start laying blame on them or yelling at them. It's not going to fix anything and it's not doing anyone any good. Take some deep breaths and stay calm. You need to be in a good frame of mind to assess the situation and do everything you need to do in terms of documenting the accident.
Here's one other thing you shouldn't do at the scene of a wreck: tell people it was your fault. If you do this, you're legally admitting liability for the accident, which could expose you to a lawsuit or other penalties. Just keep it together, make sure everyone is safe, be polite, but protect yourself at the same time.