So you would have something like:
CREATE TRIGGER trgAfterPersonInsert
ON dbo.Person AFTER INSERT
AS BEGIN
INSERT INTO dbo.Car(PersonID)
SELECT i.PersonID
FROM INSERTED i
END
The important point to remember is: if you insert 10 people into your table in a single statement, your trigger will be called once with a "pseudo-table" INSERTED
containing those ten people.
So your trigger code needs to be set-aware - do not assume the trigger gets called once for each row - that is not the case.
See the MSDN docs for CREATE TRIGGER for more details, and check out the An Introduction To Triggers tutorial on SQL Team.