First aid is care given for an injury before true medical assistance can be obtained. This can range from sanitizing and covering a wound to wrapping a twisted ankle to creating a stretcher from what is available and carrying the patient back to civilization.
There are many first aid resources on the web, but in order to learn, practice, and master first aid, you should seek formal training
While answers here may provide suggestions for first aid actions, they are not a substitute for proper training. Consider them a recommendation as to what you should learn. Do NOT attempt any first aid beyond what you have been taught by a qualified instructor.