To be able to get a man-in-the-middle position your host computer should be in the same wireless network as the mobile phone and the gateway it communicates to. Once this is done you need the IP address of your mobile phone. For a full dynamic analysis of a mobile app, all network traffic should be intercepted.
A simple way to simulate a man-in-the-middle (MITM) attack is to configure a network where all packets between the devices in scope and the target network are going through your host computer. In a mobile penetration test, this can be achieved by using an access point the mobile devices and your host computer are connected to. Your host computer is then becoming a router and an access point.
Install a tool that allows you to monitor and analyze the network traffic that will be redirected to your host computer. The two most common network monitoring (or capturing) tools are:
Wireshark (CLI pendant: TShark) tcpdump
Wireshark offers a GUI and is more straightforward if you are not used to the command line. If you are looking for a command line tool you should either use TShark or tcpdump. All of these tools are available for all major Linux and Unix operating systems and should be part of their respective package installation mechanisms.