I would like to add the following to Linc's excellent writeup, with regards to Java in item #5:
The new versions of Java distributed by Oracle have a separate Java Control Panel that opens up when you click in the Java icon on System Preferences. If you need to use Java for web access, open the control panel and ensure that the security level is set to the highest in the Security pane, as shown below:
Also make sure that it is set to check for updates automatically in the Update pane so you always have the latest version of the product. Older versions are more prone to vulnerabilities.
just wondering if there any resent trojan horse to be careful about or mac specific viruses
That depends, to some extent, on what version of OS X you are using. As Linc has said, if you are using a fully up-to-date OS X 10.6.7 or above you are protected, but each upgrade contains improved capabilities, in that respect.