In the trigger, pull out the Primary Key value from the trigger's 'Inserted' table.

select (PrimaryKey field) from Inserted

Or use the same statement to pull out the value of the column you are interested in.

Note, if your table is likely to be updated in batches, there will be more than one row in the inserted table, therefore pull the select into a cursor and cycle through the values.

see BOL "inserted tables"