This is happening because the default schema of the user who is creating the table is not dbo, but is the username. If you change the default schema (in SSMS object explorer, databasename -> Security -> Users), any new table created will use the new schema.
Alternatively, you can explicitly specify the schema name while creating the table. (i.e., dbo.MyTableName instead of just TableName).
You can transfer the tables already created to dbo as well. For example this transfers the table to dbo:
ALTER SCHEMA dbo TRANSFER [USERNAME].[TestTable]