The most common scenario is you insert into a table, then you immediately need that new ID to insert into a child table. If this is your case, and you only have one secondary insert (or update) to make, then you can just perform your two queries like so:

MySQL example from mysql.com (in the second insert, the ID is inserted in another table):

If you need that id for other purposes, then you have two choices. You can select it out using a query to SELECT LAST_INSERT_ID(), or you can use PHP's function:
[php]
mysql_query("INSERT INTO mytable (product) values ('SnippetEdit')");
$last_id = mysql_insert_id();
[/php]