Hello Laxmi,
Appointment letter is certainly important. It is the proof of the terms and conditions of employment, remuneration, etc. agreed between company and employee. But nowadays many companies do not give appointment letter immediatelty on joining because there is high rate of turnover within first couple of months. So dont worry as it has just been a month, but do ask for appointment letter if you are planning to stay with the company for longer period.
Regards,