how to target user on local network?

I have just started using wireshark. I'm using it so see how good is Internet security. But i have a problem targeting local mac, ip o computer of my choice. In aircrack, aireplay, and all others you can target computer and start capturing packets. But i haven't found same option in wireshark.

How can i capture packages, and do i need to put my wlan0 to mon0 mode? How can i target or hack or sniff computer of my choice? I have wireless connection with my router. How to see all users on it? And how to pick one out to capture it's packages?

thx