Ultimately, it will take a significant commitment from the US Government help Puerto Rico and the US Virgin Islands to recover from the devastation caused by Hurricane Maria.  In the meantime, there are some ways for civilians to help.

PBS has a good list of charities.  Foundation Beyond Belief is raising funds for recovery efforts for all three hurricanes this season.  You can also donate to the Red Cross, though it might be better to donate to a local relief organization instead.  Charity Navigator offers research on many charities, as well as these other sites.

Wherever you donate, consider giving cash instead of supplies.  Money allows organizations to buy the supplies they need.  Material donations take time to sort, and not all donations can be used.

Again, it will take a significant commitment from the US Government to rebuild Puerto Rico and the Virgin Islands, but donating now can provide some relief.