This is one of the few "absolutes" of giving care.  If a victim has an obstructed airway, cannot breathe, or has no pulse, nothing else matters!  It may be hard to ignore other severe injuries; blood certainly commands attention, but don't let it distract you.  You may do a great job of bandaging them up, but if they die because they couldn't breathe, you will have accomplished nothing.

Assessing  the ABC's

If the victim is unconscious, look, listen, and feel for breathing.  If they are not breathing, give  Rescue Breaths, if they are breathing, watch them closely for changes in in breathing or consciousness while you wait for the ambulance.

If the victim is awake and can speak, you can assume that they are conscious, breathing, and have a pulse.  Don't get too relaxed, though.  You will want to monitor their airway and be prepared to take action, as this can change.  If the victim is conscious, you must obtain permission to care. This means that for them to make an informed decision you have to tell them who you are, your level of training,  and what you intend to do for them.  If they refuse, which they have every right to do, not attempt to give any further care. 

There are certain situations where permission is Implied:

The guiding principle here, is that you may assume that if a victim were able to give permission, they would.

While we are on the subject of legal issues, you should familiarize yourself with the Good Samaritan laws, which protects people providing emergency care.  In general you are protected from legal liability as long as you act in good faith, are not negligent, give care up to but not exceeding your level of training, and do not accept any form of payment for said care.  These laws vary depending on your location, so consult a legal professional or your local library for specifics.

Once you have permission, care for life-threatening injuries first.   If there are no life-threatening conditions, you can move on to checking for other injuries. If the victim can't pin down what hurts check for injury methodically from head to toe, asking questions as you go. In the case of a child, they may feel less threatened if you check from toe to head. Open ended questions are a good idea when caring for children. (e.g. "Can you point to what hurts?" instead of "Does your arm hurt?"often yields a tearful "Uh-huh" even though the injury is on their knee.)