func ServeHTTP(w http.ResponseWriter, r *http.Request) {
    w.WriteHeader(http.StatusInternalServerError)
    w.Write([]byte("500 - Something bad happened!"))
}

func yourFuncHandler(w http.ResponseWriter, r *http.Request) {

    http.Error(w, "my own error message", http.StatusForbidden)

    // or using the default message error

    http.Error(w, http.StatusText(http.StatusNotFound), http.StatusNotFound)
}

w.WriteHeader(http.StatusInternalServerError)
w.WriteHeader(http.StatusForbidden)