The best thing Microsoft ever did for security was turn on the firewall by default way back in the days of Windows XP, Service Pack (SP) 2. A firewall is a program that restricts access to (and from) your computer. It makes your computer much more safe, and should never be turned off for any computer connected to the Internet. Before XP SP2, the Windows firewall was turned off by default, meaning users had to know it was there, and turn it on themselves, or be left unprotected. Needless to say, many people failed to turn on their firewall and had their computers compromised.
The firewall in Windows 7 isn't much different, technically, than the one in XP. And it's just as important to use. As with all later versions, it's on by default, and should be left that way. But there may be times it has to be temporarily disabled, or for some other reason gets turned off. That means learning how to use it is important, and that's where this tutorial comes in.
To find the firewall, left-click on, in sequence, Start/Control Panel/System and Security. That will bring you to the window shown here. Left-click on "Windows Firewall," outlined here in red.