a good thing to do for a first time fish owner is to do some reaserch before getting a fish. Just like studying for an exam.
what is the tank size
what are the water parameters (tested with a liquid test kit)
do you have a heater for your betta? (those are needed for bettas need 78 degree temperature always)
it sounds like he could have some tail rot. it would be even better for a pic of the fish
more good info is in the betta gereral care sticky at the top of the anabantids forum