Check with the CDC (link) for immunizations and anti-malarial prophylaxis (medicine to keep you from getting malaria). I used Doxycycline which is cheap, but has to be taken every day and makes one more sensitive to the sun.
It was simple to get my visa at the airport. I had a letter of invitation, but I don’t think I needed it. Cost was $40US, and they had change for a $50 at the desk. Most people recommend getting the visa ahead of time via TravelDocs or a similar service — it would have saved a lot of time at the airport.