I'm assuming you want to sniff 802.11 traffic, right?
Use Wireshark in Monitor Mode. When you're the menu, there should be a cog. Tick the box near Monitor, then start sniffing. You should see beacon frames and things. Also, I've written a little guide below to help you decrypt the traffic.
[Disclaimer: Please use this for educational purposes. I am not responsible for any damage you use this for. Sniffing 802.11 packets is legal as long as it has no criminal intent.]
To decrypt the traffic, you must have an 4 way handshake (EAPOL) and the WiFi password. See some things below.
Tip: Preferences > Protocols > IEEE 802.11 > Decryption Keys > (+)
The key type would be WPA2 or WEP for example. The key would have the SSID and the password.
For example, you would have it in the format of PASSWORD:SSID. Make sure to enable decryption!