You can follow these steps to connect your Samsung watch to WiFi:
Press the bottom button on the watch.
Select the "Settings" icon. This is the one that looks like a gear.
Scroll through the options till you find the "Connections" option.
Scroll through the options till you find "WiFi" option. The, select "WiFi networks".
Select your WiFi network. Use the keypad on the watch to input the WiFi password.
You can also follow along with this video to connect your Samsung watch to WiFi.