In case you need to configure Pi as an access point.
Here is official guide: https://www.raspberrypi.org/documentation/configuration/wireless/access-point.md
If you have a pi without build in Wifi and you got Edimax USB Wifi,
you might need to compile your own hostapd with appropriate driver:
https://github.com/pritambaral/hostapd-rtl871xdrv