You can connect an Arduino to the internet using an Ethernet shield or a Wi-Fi shield.
For example, the Arduino Ethernet shield allows you to connect your Arduino to the internet using an Ethernet cable. You can use the Ethernet library to write code that will allow your Arduino to send and receive data over the internet.
Another example is the Arduino Wi-Fi shield, which allows you to connect your Arduino to the internet wirelessly. You can use the Wi-Fi library to write code that will allow your Arduino to send and receive data over the internet.