Getting SQL*Net Running on Oracle

We have been trying to get SQL*Net running but with no success. tnsping works fine. When we use SQLPlus to connect, we get the following error :
ERROR: ORA-12500: TNS:listener failed to start a dedicated server process

Any idea what we are doing wrong or have not done???

We also did not find any documentation which precisely tells what needs to be done to get SQL*Net running. Is there any such documentation available out there? Any FAQ?

Any help would be highly appreciated...

TIA,
Nirmalya

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