First things first: Immediately upon noticing an accident, report it to officials. Never think "someone else has probably already done that". I've arrived on a traffic accident site three times in my life. In two of those occasions there were already other bystanders there, but all of them were so disoriented that none had called 911.
As for the risks involved, the obvious ones are possible fire / explosions on site, and the collapse of the wreckage. The fire is most likely to ignite upon impact, and planes seldom carry other volatile material than fuel. Secondary explosions are still possible due to pressure vessels, tires etc. I may also be that the crashed plane is a military aircraft, which may carry ammo. On a site like the crash you mentioned, landslides are a possibility also.
Assessing the risks mentioned above may be difficult for a layman, but should one decide to help, I think this is not much different from any other accident type. There is some variation between instructions, but the main steps are:
- If there are survivors, assist them. Get the injured safely away from the wreckage if remaining there poses a danger. Give first aid to those in need.
- If there is a fire and you have the means, try to suppress it.
- When the officials arrive on site, do exactly what they tell you to do.
All in all the main thing is to remain as calm and collected as possible, and do not take unnecessary risks. Doing this will not be easy.
Aside from the above, do not touch anything on the accident site unless it is necessary for saving lives.