If you want to find schema name for a particular object (table/index/view/procedure etc), then use the following (owner column is the schema name) -

select * from user_objects where object_name = '<objectname>';
or
select * from all_objects where object_name = '<objectname>';
or
select * from dba_objects where object_name = '<objexctname>';

where
- objectname is the table name or index name or procedure name.
- user_objects is used when finding objects in your own schema.
- all_objects is used when finding objects in all schemas for which you have been granted access
- dba_objects is used when finding objects in all schemas in the database

If you want all the schema names in the database and if you have DBA access then use -
select * from dba_users;

Hope this helps.