Oracle Database Discussions

I set up an Oracle server on a Linux box and installed fine.
Created a database, post-installation of Oracle, and listener.ora recognizes the service with the lsnrctl status and service commands.
I can startup / shutdown with no issues from sqlplus.
I have installed Oracle many times but coming back to it after a hiatus since 10g.
So unsure as to how the automatic service registration happens and my questions are around that.
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So when I go to set up a connection to the db - either locally or from a client using Netca I get the following error:
Connecting...ORA-12514:
TNS:listener does not currently know of service requested in connect descriptor
The test did not succeed.
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So I tried putting the instance info in the listener.ora file - as I have done before and also in examples - but keep getting the error.
But as I mentioned - lsnrctl status and services gives that the service is running.
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I am reading that in 11g the service automatically registers - which it does in my case. So the bog question is:
How do I set up a connection to it. I have been told I should directly edit the tnsnmaes.ora file (as I have also done before) but i just wanted to find out the right way to do this for 11gR2.
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I am reading something about the PMON process registering the service and entries in the init.ora file but I have not found anything definitive as to what parameter needs to be where.
As I mentioned I created the database post-installation of Oracle.
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It would be great if someone could help me with what exactly I am missing - as this is frustrating because this was a simple thing before. Morover since this is such a critical part of connecting to the db I am befuddled that I just cannot find any concrete reference to exactly what needs to happen.
Like I said I tried putting in the instance info in the listener.ora file but no connection yet.
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Would really appreciate your help in understanding the right way to do this and not hack the listner.ora or tnsnames.ora - if possible.

Thanks much.