With the reopening of Kabul International Airport to civilian traffic, the easiest way to Afghanistan is by air. On touching down there's a (mostly) orderly queue for immigration, but as a foreigner you'll often get fast-tracked. After this, there's a separate booth for passport registration, before the scrum of the baggage hall. Customs checks are mostly in name only.
Officials are similarly laid-back if you're entering overland. Most of Afghanistan's borders are so porous, you'll be generally looked on as more a novelty than a bureaucratic hindrance. Bear in mind that Afghanistan has 30 minutes' time difference with all its neighbours. Some borders are closed on certain days, and everything grinds to a halt on Friday lunch time.
The national carrier, Ariana Afghan Airlines, Ariana offers a comprehensive domestic service, but demand definitely outstrips supply, so book as far in advance as you can. A private airline, KamAir, offers an alternative service. Neither hold fantastic safety records.
Failing an airline seat, your next best bet is a minibus; Afghanistan is held together by the indestructible fleet of Toyota HiAces that ply the attrocious roads across the country.
Yellow-and-white shared taxis are a popular way to travel between towns. They leave from the same terminal as minibuses. They're faster than minibuses and, with fewer seats, fill up and leave sooner. Fares are around a quarter to a third more expensive than the equivalent minibus.