If you're in a place that already has a WiFi access point set up (a wifi router) and it's free (most Starbucks, Mcd's, some bookstores) will have free wireless that you can use one of those wifi adapters that I linked to. You dont need a plan with that kind of adapter, because it's made to just access free internet spots.

If you're in a place where they dont have any wifi offered, or if you dont have your own wireless internet router at home then you would need to get an adapter that works similarly to a cellphone, except you dont plug it into a cellphone, you plug it into your computer. Most cellphone companies offer those now. You plug it into the computer and the computer uses it to connect to the cellphone towers/satellites to get you on the internet.