Listener ... giving hard time

It's always the case, don't know what error or what configuration mistake I did and then all of sudden, it starts working too, surely after doing some tries, different options & searches too, then it always start working too.

What I'm looking for over here is a simple 1,2,3 steps, as when created a new db, NOT on 1521, what I should be doing to make it work from client application too, where as able to ping the server, able to tnsping too, but when trying sqlplus, getting these errors:

ORA-12514: TNS:listener does not currently know of service requested in connect descriptor