1) Mac security problems may be rare, but the computers are not immune to viruses. There is some debate about whether Macs are inherently safer than PCs or if hackers simply target PCs because there are more potential victims. Either way, using a Mac should not make you complacent about security,
2) Don’t install pirated software. Even leaving aside the legal and moral issues, it creates a serious security risk as you have know way of knowing whether they have been tampered with.
3) Even if you choose to ignore point 2, you should never type in your system name and password while using any program that is not 100% official.