First aid means an immediate care given to an ill individual. Since first aid can be lifesaving, it is important to know some basic principles. First aid includes a wide range of treatments, such as critical care, tending to an abrasion and performing CPR. Sometimes, a person has to perform first aid treatments until medical help arrives.
Of course, an attempt to help is better than doing nothing, but without knowledge and skills, the idea of helping an injured person can be frightening. What you can do is make sure you don't make everything worse. If you don't know how to perform a treatment, don't do it. For example, you shouldn't move a trauma victim unless not moving that person puts him/her at higher risk. Don't remove an embedded object as you may do more harm. And if there is nothing you can do, you should call for help.