Devices do not give off Internet signals. Devices, such as wireless routers, accept a connection from an ISP over some form of physical connection (copper or optical) and they generate a WiFi signal.
You can buy a wireless routers and legally use that to give off an unlimited WiFi signal, which you can use connect local devices together and provide your own private local network. You still have to pay for the ISP connection that delivers the Internet to your router.
As mentioned in another answer, some communities provide local short-range WiFi that you can use for free, This will give you an Internet connection and you don't have to buy anything as the community hotspots are giving off the WiFi. Some coffee shops and fast food outlets provide free WiFi, but you are expected to buy something (a coffee, a burger or something) before you use their WiFi during that specific visit.
Anything else is potentially theft, and can lead to prosecution, a fine or even imprisonment.