There’s a long list of countries whose citizens don’t require a visa to visit Botswana for up to 90 days. If you’re from Europe or North America, you almost certainly won’t need one. Just make sure you have at least two blank pages left inside your passport and that it’s still valid for six months from your date of departure.
Most Commonwealth countries also have a visa exemption, but if you’re not a citizen of one of these countries, or wish to stay longer than 90 days, then you’ll need to apply for a visa from a Botswanan embassy or diplomatic mission. Costs vary depending on your nationality and the duration of the visa required.