How can I reset the auto_increment id field in my table?

First, delete all records in the table (assuming you want to start at 1).  Then run this command:

ALTER TABLE tablename AUTO_INCREMENT = 1

Note:  If you don't delete all records first, this will reset the next number to 1+ the highest number in your table.  This might be useful if you remove a bunch of records at the top end of the table.