type MilvariableStartswithUpperCase struct {
    Id  int64  `json:"id"` //sss
    Cty string `json:"firstname"`
}

type MilvariableStartswithLowerCase struct {
    id  int64
    cty string
}

upperCase := MilvariableStartswithUpperCase{1, "UpperCase"}
lowerCase := MilvariableStartswithLowerCase{1, "Lowercase"}

marshalsucessfull, err := json.Marshal(upperCase)
if err != nil {
    fmt.Println("error:", err)
}

marshalunsucessfull, err := json.Marshal(lowerCase)
if err != nil {
    fmt.Println("error:", err)
}

fmt.Println("----")
fmt.Println("Marshal Sucess")
os.Stdout.Write(marshalsucessfull)
fmt.Println()
fmt.Println("Marshal Failed.")
os.Stdout.Write(marshalunsucessfull)