Make sure you know with whom you are dealing.  Also consider what information they want, and if it is reasonable for what you want from the site.  If you're not sure, look to see if it is certified by an Internet Trust Organization.  (See also, what is an Internet Trust Organization?)  If it is a retailer you don’t know, you might want to use a free third party payment tool like PayPal, an eBay company.  PayPal protects you by allowing you to pay retailers through a PayPal account linked to your credit card or checking account, instead of giving your account information directly to a retail Web site you never heard of.  PayPal also offers an optional personal security device, a very safe way to protect yourself when paying online.