What OS are you running the Bitcoin Core on?
You have to create a file called Bitcoin.conf in the directory that is in the same place as your blockchain database. In the file, you have to include these:
server=1
rpcuser=SomeUserNameButDon'tUseThis
rpcpassword=ARandomAndSecurePasswordAndTodayIs8/7/2017
Replacing the rpcuser and rpcpassword accordingly.