How to make formula field for salesforce field?

I am trying to make formula field for the salesforce field. the condition is given below.

if Company = "WIL" And (ShippingCountry = "United States" Or "USA") then
   "US"
elseif Company = "WST" And (ShippingCountry = "United States" Or "US") then
   "USA"
elseif ShippingCountry <> "" then
   ShippingCountry
elseif Company = "WIL" then
   "US"
elseif Company = "WST" then
   "USA"
else
   ""
end if