Papua New Guinea → Monaco: Short-stay tourist

Monaco does not issue its own tourist visa. If your nationality is not exempt, you need a Schengen short-stay visa, applied for at the embassy or consulate of the Schengen country that will be your entry point into Europe, in practice a French consulate for most Monaco trips, since Monaco's only land border is with France. Once you have the Schengen visa, visiting Monaco itself needs no extra formality. Monaco's page does not publish the visa's own document checklist (photos, proof of funds, travel insurance, itinerary, and similar) since the issuing Schengen consulate sets that, not Monaco, confirm the exact list with that consulate.

Source: Prince's Government of Monaco, MonServicePublic.mc

Foreign nationals staying in Monaco for less than three months need a passport to enter. Monaco's page does not publish its own minimum validity period for the passport itself, the Schengen consulate issuing your visa will set that, so check with them directly.

Source: Prince's Government of Monaco, MonServicePublic.mc

If you need to prove accommodation in Monaco for your Schengen visa application, for example you're staying with a private host rather than a paid hotel, Monaco's own Police Department can issue a Certificate of Accommodation for a Tourist Stay. Apply in person at the Residency, Certifications and Lost-and-Found Section with the application form, the host's lease or ownership deed, the host's ID document, and a copy of your own ID; the fee is €5 in Monegasque tax stamps, paid after a 48-hour checking period. The certificate must then be validated by the Consulate General of France for your place of residence before it supports your Schengen visa application.

Source: Prince's Government of Monaco, MonServicePublic.mc (Police Department)