Once the wave of first responders ensures that the situation in Puerto Rico is stabilized, the need for additional volunteers will increase. The Federal Emergency Management Agency directs aspiring volunteers to register here, and you can also look at a list of opportunities at the National Voluntary Organizations Active in Disaster. You may not get the call straight away.
Countless others have similar desires to volunteer in Puerto Rico, but it's important to consider that the battered island will need help in the months — if not years — to follow. A simple post on social media or discussion with friends or co-workers can go a long way. Some people are still not aware of the extent of the damage, so any method you can use to spread the word may be able to convince others to help as well.