A fair amount of public locations, such as businesses and shops, have wireless networks associated with them. While many of these networks are unprotected--meaning you don't need a password to access them--they are also often insecure networks with minimal protection. This is the series of radiating waves in the top right corner of your screen. If you're in an area with wifi capability, you'll see at least one network name. If you have the option of picking a password-protected network over an unprotected one, go with the protected network--being able to connect to a secure network is worth the hassle of having to ask an official (e.g., shop staff) for the password.  Depending on your location, you may have to pay a fee (or purchase merchandise) before you can use a protected network. If you end up choosing an unprotected wifi network, be careful about the information you view and input while using it. Other users may be able to view this information as well. Generally speaking, the network in question will have the strongest signal out of all the surrounding networks. If the network has a password, you'll need to type it in first. Only do this if you use this network often. You should now be connected.

Summary:
Log into your Mac. Click the wifi icon. Review the network list. Choose a protected network. Click the pertinent network. Click Join. Check the "Remember this network" box.