First, you have to think of your own safety. Check yourself for injuries,
Park your car well off the roadway.
Make sure that. other drivers have time to react before making things worse. Always turn on your emergency flashers.
Take a few moments to gather yourself. If you have witnessed a serious crash, your body will be dealing shock. your heart will be racing.
Taking those few extra moments, to collect yourself, before acting will make you much more effective. If you have a cellphone, call 911. Tell the dispatcher where you are and what you can see.
Remember, your job is to hold down the scene until the professionals get there. If it is safe, make your approach, but keep looking for things that are immediately life threatening — like fire or ruptured fuel tanks. Do not start any First Aid beyond any training you may have had.