Visas: Most nationalities require a visa. Visas on arrival are available at Bissau airport, but it is safer to arrange one in advance at a Guinea-Bissau embassy if you can find one (they are scarce). Your passport must be valid for at least six months beyond your arrival date.
Yellow fever: A yellow fever vaccination certificate is mandatory for all travelers aged one year and older. You will be asked for it at the border and it is strictly enforced.
Money: The currency is the West African CFA franc (XOF), shared with several other countries in the region. Bring euros or CFA francs in cash -- there are very few ATMs in the country and they are unreliable. Credit cards are almost never accepted outside the rare upmarket hotel in Bissau. Bring more cash than you think you will need.
Language: Portuguese is the official language, but Kriol (a Portuguese-based creole) is what most people actually speak day to day. French is occasionally useful near the Senegalese border. English is rarely spoken.
Safety: Guinea-Bissau has experienced repeated coups and political instability since independence. Check the current situation before you travel. Street crime exists in Bissau, particularly at night -- take the usual precautions. Landmines from past conflicts remain a risk in rural areas, especially in the regions of Bafata, Oio, Biombo, Quinara, and Tombali. Stick to established roads and paths.
Health: Medical facilities are extremely limited. There is no reliable emergency care in the country. Carry a well-stocked medical kit, take malaria prophylaxis, and drink only bottled or treated water. If you have a serious medical issue, you will likely need evacuation to Dakar or Lisbon.
Electricity: Power supply is erratic. Many areas have electricity for only a few hours a day, and blackouts in Bissau are routine. Bring a headlamp and a power bank.
Communications: Mobile phone coverage exists in Bissau and major towns (Orange is the main provider), but is patchy elsewhere. Internet is slow and expensive. Do not rely on being connected.