What determines the default schema name in SQL Server? when I tried to create a table, it is using my ID as the schema name. But when the table is created on my manager's computer, the schema name is "dbo". Why? He might have administrator rights. When I tried to rename by using sp_rename, it is not changing.
In SSMS go to your database user and click on properties.  You will see a field there for default schema.

T-SQL version:

alter user YourDbUser 
with default_schema = YourNewSchema