You will need a valid tourist visa for Myanmar. You can organise an eVisa (visa on arrival) before you travel to Myanmar at http://evisa.moip.gov.mm or by contacting the Myanmar Embassy in your home country. Your travel agent can also help arrange your visa.
In order to access Lampi, special permits need to be arranged for you by a tour agent. You should book your tour through a licensed liveaboard tour operator at least 2 weeks in advance in order to obtain the travel authorisation in time for your trip.