Character Sets are determined at the time you create your database and are set for the entire database, not a single user. It is possible to migrate from one character set to another using export/import if conditions are correct.
Here is the documentation link that explains it. I would post more info but you will get it when you read the documentation. Unfortunately, character sets are a sensitive component to oracle and there is no quick answer. You will need to read the documentation.
You will notice there is a section on how to change your character set.
Good luck, Kevin