As default VanPI will boot an AccessPoint (SSID: 'VanPI_xxx', passphrase: 'vanpifetzt' - On versions older than v1.0.0 it is SSID: 'PekawayControl_xxx' with password 'pekawayfetzt').
You can connect to it via any device you want. To reach the web-interface, simply go to 'http://van.pi' (below version v1.0.0 use 'http://peka.way') or 'http://192.168.4.1:1880/ui'. (Important: Most Android phones will ask you if you want to keep the connection alive, as it is not connected to the internet. You will have to keep that connection.)
Join existing Wifi
When you want to join an existing wifi go to the wifi settings and click on 'Get available wifi'. The RPI will then turn off the access point, reboot, scan for wifi networks, turn back on the access point and reboot again. This can take up to several minutes.
You can then choose the desired network from the dropdown menu right beneath said button. Insert your credentials and let the system reboot one more time. It should then connect to the existing wifi and you'll have to access the frontend with it's new ip-address.
The system will check if it got a valid ip-address from the DHCP server, if not (e.g. wrong passphrase - no proper network connection can be established), then it'll reboot again and turn back to access point mode.
If you're connecting to an exisiting network via cable, the eth0 port will be bridged to wlan0, so that everything that is connected to the RPI's access point will be able to access connections via the eth0 port too.