In case you do see a teenager or adult suddenly collapse, these are the steps to follow:
Step 1: Call 911
Step 2: Push Hard and Fast in the Center of the Chest
Begin pushing on the center of the chest hard and fast until help arrives, keeping a rhythm of 100 compressions per minute. If you start to feel tired, someone else can take over. But whatever you do, don’t give up -- you might be the only chance this victim has.
Step 3: Don't Stop Until Help Arrives
Note: Hands-Only CPR can be used in place of conventional CPR (CPR with compressions and breaths) for victims over 8 years old. Conventional CPR should be used for anyone under the age of 8 years old, or for any aged victim of drowning, drug overdose, or a collapse resulting from breathing problems. Anyone over the 8 years old who you have not seen collapse should also be given conventional CPR.
Source:bethebeat.heart.org (Hands-Only CPR video from the site, available here), handsonlycpr.org