Security for e-commerce
As far as security for e-commerce goes...
Install (and run regularly) some anti-spyware programs on your PC. Some good free ones are "Lavasoft Ad-Aware", "Spybot Search and Destroy" and "PestPatrol". This prevents any spyware illicitly and unknowingly installed on your PC from recording your user ids, passwords, credit card info, etc as you type the keystrokes.
A second thing to do is only use e-commerce sites that encrypt all sensitive information (especially the credit card payment info). The way to tell that encryption is active is to look at the address field at the top of your browser - the URL should begin with "https://" instead of "http://" (the 's' is for secure sockets). Also, you should be able to see a lock icon on the lower right of your browser while secure sockets are in use. That protects the information on it's trip over the internet between your PC and the company's servers.
If the site is a small one, that might have unsophisticated security, you should also review their security policies - they should not store your critical credit card information in their databases, which can be hacked.
With proper precautions, e-commerce is quite safe.