error creating trigger

I created a database, but when I run a script to create a trigger, I get error:Cannot create the trigger 'MyDDLTrigger', because you do not have permission.what permissions do I need to set to create trigger ?Here is part of the script that seems to be causing the error:CREATE TRIGGER [MyDDLTrigger] ON ALL SERVER WITH EXECUTE AS SELF FOR DDL_DATABASE_LEVEL_EVENTSAS...............GoThanks.