Hi
If you mean the ip visible to the outside world you normally need your isp to set this up.
If you mean that you want a device to always have the same ip address on your network (as is useful with printers) then normally in your router settings you can associate a specific ip with a specific wifi mac address(each device has its own Mac address). You then need to reboot your router and give the specific device first choice of ip addresses.
Hope this helps
Roger