If you want a secure site with any degree of credibility, you'll need an SSL certificate issued by a well known and respected authority like :
http://www.verisign.com/products-ser...ssl/index.html
This will cost a fair amount mind you, but the investment is worthwhile if you plan to make money from your site.
If you don't know how to set up a secure server, I suggest you talk to your hosting company as they may well have a preferred route.