If you want to sell your Chametz permanently, without the hope or desire of ever having it again, you can simply sell it to any non-Jew who wants to buy it. S/he should give you the money, you give him/her the Chametz, and everyone is happy.

If, however, you have a pantry or freezer full of Chametz, not to mention a china closet full of Chametz dishes, and you would obviously like to have that back, you must sell your Chametz through a Rabbi who executes a complex nuanced sale with a non-Jew. The Chametz you wish to sell should be stored/covered and that area should be sealed off. The sealed off area of Chametz will be leased to the non-Jew as part of the transaction, and you may not open/enter that area.

The Rabbi sells the Chametz on the morning before Passover, and buys the Chametz back on the night after Passover. The Rabbi needs to have your "sale of Chametz" form before he executes the transaction, and you may not use the Chametz until the Rabbi buys it back.

Preferably, your local rabbi should handle the selling of your Chametz. If you do not have access to a rabbi you can sell your Chametz online at Passover.net.