For the most part, just follow these three steps:
1) Turn on the firewall. (In system prefrences under “Sharing.”)
2) Surf smart. Don’t download/open files from the ‘net if you can’t say for sure where they came from.
3) Under ‘Advanced’ turn on “Stealth Mode.” You’ll end up enternig your password more often, I think, but that’s a good way to make you slow down and think before doing something.
That’s probably enough to keep you safe. If you’re still worried, make a second account under “Users” in system prefrences and DON’T make it an admin account. Log into your “main” account to load software and run updates, but use the second account all other times. That’s an extra layer of security that I don’t follow, but it’s an option if you feel like it.