Could be a lot of things, but first thing's first. Tell us phone model, carrier (AT&T, Verizon, O2, etc.) and what Android version you have (go to menu>settings>About phone).
You say you are a new user ... how long have you had the phone? Brand new phones some times mis-report the charge level so while you think you had a full charge it might have been only partially charged. Now that it has discharged to zero, plug it in using the AC adapter and let it charge to full ... unplug the phone when done and turn it off. Then plug it back into the charger and let it go for at least another hour. The new lithium batteries do not need to be "conditioned" but you do need to educate your phone to what a full charge really is. In the future if you notice battery issues, a deep discharge cycle can reset the values the phone looks for, but constantly letting the phone go to zero can damage the battery.
One other note, if you installed a task killer (even if you were told by the store staff that you needed one) uninstall it. They can kill a battery faster than Tiger Woods can kill a marriage.
