a foreign key is a primary key in another table, so it should already be autoincremented in the table where it is the primary.
your other question about having assigned id's reuse deleted id's actually requires a little more work. once an id has been used, it is removed from the range of id's to be auto assigned. so you can either manually assign the id at the time of insert by using a subquery to add one to the max() value in the field in the table.
THIS WILL DELETE ALL DATA IN THE TABLE DO NOT DO IT UNLESS THAT IS WHAT YOU WANT
If you're starting from scratch and scrapping all of the data in the table, and just want to begin with one instead of whatever the next number was; you can truncate the table (either using truncate table command, or select the table in phpmyadmin and go to operations, and select truncate).