You can fly into Lusaka (Zambia) or Lilongwe (Malawi). If you are coming as a qualified volunteer the hospital will usually arrange for someone to collect you from the airport. If you are a medical student then you will need to make your own way!
Most visitors fly into Lusaka (Zambia’s capital), and then travel by bus to Katete - a 5 - 7 hour ride along the Great East Road heading towards Chipata and Malawi. It is best to take an early bus, as the roads can be risky after dark when it’s more difficult to see other road users (cars, bikes, cattle). So unless you’re a hardy African traveller, consider spending the night in Lusaka then buying a bus ticket in advance ready for travel the next day. The bus goes past SFH’s main entrance road (about 7km before Katete): the driver should know it, and will stop there if you ask in advance.
Alternatively, if you fly in to Lilongwe (Malawi) then take a bus to Mchinji, a taxi to the border, then another taxi to Chiapata and finally a minibus to Katete. It’s a shorter distance, and a more colourful way to travel, but considerably more hassle.