Mac OS X Built in Firewall

It will ... turn the firewall on? I'm not sure what you're asking. For most folks, I can't imagine getting into trouble turning it on, but it depends on the apps you use, if you're running anything that would be considered a server over TCP/IP, etc.

 

Nothing will happen, you'll be... safer i guess

 

The built in firewall will give an extra layer of protection behind the router firewall, nothing drastic will happen to your mac if you turn it on, you'll just make it a little more secure.

Once the firewall is turned on use your mac as normal, if everything works as normal there is no need to make any further changes, if an application that requires internet or network access isn't working correctly use the advanced tab to allow access for specific applications,