You need to purchase an SSL certificate - most hosting providers sell them. SSL certificates are similar to domains in that they are only valid for a short period of time (usually 12 months) and then have to be renewed.

Web browsers come with a list of SSL certificate authorities that they automatically trust, and therefore you need a certificate that is trusted by the major browsers (which will ensure that visitors don't get a dialog box popping up when people visit your site).

You can get various different types of SSL certificate with different levels of browser compatability, and that verify different things (e.g. your identity or address etc.). Some also include an insurance policy that covers any fraud that may occur due to insecurity of the connection - this enables you to display a "button" graphic to reassure your customers.

Make sure you take a look at the different SSL certificates available and select the most appropriate one for your purpose.

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Damien Ransome
layershift :: UK & USA-based Managed Dedicated, DDS, VPS, Reseller & Shared Hosting
Experienced SWsoft Platinum Partners (Plesk since 2001, Virtuozzo since 2004)