What's your question got to do with a trojan horse? If you have a trojan on your computer, run your anti-virus and remove it.
As for on websites; only ever give details like your credit card information to those sites that use https, with the padlock at the bottom.
Also, choose good passwords for your accounts. If you use a program like
PasswordSafe to remember your passwords for you, then let that program generate a random password for you. If you keep your passwords in your head, then choose a really weird phrase that you can easily remember like "some ducks are pink".