Getting to Afghanistan is difficult and requires serious advance planning. Most countries advise against all travel, and standard tourist infrastructure does not exist.
By air: Kabul International Airport (KBL) is the main entry point. A handful of airlines operate flights, primarily from the Gulf states and Pakistan. Kam Air and Ariana Afghan Airlines run limited services to Dubai, Islamabad and a few other regional destinations. Flight schedules are unreliable and change without notice. There are no direct flights from Europe or North America.
By land: Overland crossings exist with Pakistan (Torkham near Jalalabad, Chaman near Kandahar), Iran (Islam Qala near Herat), and the Central Asian republics (Hairatan near Mazar-i-Sharif). Border crossings may close without warning. The security situation along most land borders ranges from uncertain to dangerous.
Visas: The Taliban government issues visas, but the process is opaque and varies. Some travelers have obtained visas on arrival at Kabul airport; others have arranged them through Taliban consulates in Pakistan or Iran. Requirements change frequently. There is no reliable online application system. You should contact a specialized travel agency with current Afghan experience before attempting entry.
A word of honesty: Afghanistan is not a country you enter lightly. If you go, you need a clear plan, local contacts, and an acceptance of the risks involved. Most travel insurance policies explicitly exclude Afghanistan.