So at work I have been given a web application server that uses Oracle as the backend. I know nothing about Oracle, I have been working with Tech Support for the application but am unable to get oracle to allow me to login.
I get an error 12514 when trying the application, and when trying to log in to the database. I have checked the TSANames.ora and Listener.ora and they refer to localhost so it appears to be right per support.
Glad it helped. He did a good job on explaining troubleshooting and tools. Next time post more information and we may get you to the "end-zone" sooner!