If you come across someone who is unconscious, there are a few things you can do to help, even if you haven't had any formal training. The important thing is to stay calm, and don't try to handle anything beyond what you know you can do, while remaining confident in your abilities (I know, this sounds like a paradox). Basic first aid courses are invaluable and normally not very expensive; knowing what to do in a medical emergency will put you on auto-pilot and make treating a victim that much easier.
Survey the Scene
Before you attempt to do ANYTHING, you need to make sure the situation is safe for you to approach. Your own safety has to be your priority; you don't want to get injured yourself. Take 10 seconds and look around, see if you can determine why the person might be unconscious-did he fall? From where? What else could have happened? Is there anything going on that could put you in danger if you were to approach? Be aware of your circumstances.
Check for Consciousness
Once you have determined that it is indeed safe to approach the person, check and see if the person really IS unconscious. Tap or shake the person (firmly, but not violently) while giving verbal cues: "Hey! Are you ok?" Basically what you are looking for here is any kind of response. It's possible the person is just sleeping. Or they could "come to," in which case your course of action will change. If you do not get a response, now is the time to call in emergency services.
Call for Help
It is perfectly ok to tell anyone else at the scene to "Call 911! Now!" if you feel comfortable proceeding with first aid. Having someone else be the one to call for help frees you up to further evaluate the person's condition. Alternatively, you can call for help yourself, but you should not leave the victim unattended if you can avoid it. If your area does not have 911 services, it is a good idea to have some type of ambulatory/emergency service phone number programmed on speed dial into your cell or mobile phone.
ABC stands for airway, breathing, and circulation. This will tell you a good amount about your victim, and will also be useful information for the EMTs when they arrive. The person should be lying on his/her back for this evaluation. Check the airway and make sure it is not blocked. Open and look inside the mouth. If you can see something in the mouth, you'll have to reach a finger in and scoop it out. Look, listen, and feel for breathing. Look to see if the chest is rising and falling; listen with your ear close to the person's nose and mouth, and see if you can feel the person's breath on your cheek (you can do this all simultaneously if you position yourself to the side of the victim, leaning over him/her, ear to their mouth and looking at their body). Feel for the person's pulse, usually most easily found in the carotid artery in the neck.
What to do if...
If the victim is breathing on his/her own and has a pulse, don't leave! If you have a blanket on hand, cover the person, and also insulate from the ground-body heat is lost much faster through direct contact with a cold ground than by a cool breeze (if you suspect heat stroke or heat exhaustion, skip this step).
If the person vomits, roll them gently to their side so they do not choke.
If the person is not breathing BUT has a pulse, give rescue breaths if you know how. Pinch the nose, create a seal with a breathing barrier if you have one, or your mouth on theirs if you don't, and breathe into the victim, watching the chest to make sure the breaths go in.
If rescue breaths do not go in: perform the Heimlich maneuver if you know how. Chest compressions are also acceptable in this situation in an effort to remove an object lodged in the airway.
If the person is not breathing AND does not have a pulse, perform CPR (Cardiopulmonary Resuscitation) IF YOU KNOW HOW.
Continue to monitor the victim until help arrives. Having training in CPR and basic first aid will prepare you for many emergencies, as well as giving you the confidence to help when the situation arises. But even if you aren't trained, you can help a person in need if you remember at the least how to (a) survey the scene, (b) check for consciousness, and (c) call for help.