You need to have a passport for the country you are a citizen of, and go to the embassies or consulates for all the countries you plan to visit. They will stamp your passport with a visa that’s valid for entry for up to some number of months, and valid to stay there for some number of months after that. You may be able to mail your passport, but if the embassy or consulate is close by, you’re better off taking your passport in person and waiting. There will almost certainly be embassies for these countries in the capital of your country, and may be consulates in other major cities. Some smaller countries don’t have their own consulates, but will have “consular sections” in consulates of countries they’re friendly with. But before you do any of this, if you’re using a travel agent to arrange your itinerary, ask your travel agent.