Most visitors (apart from people from a few visa-exempt countries in Africa) will need to get a visa to Uganda. You used to be able to get a visa on arrival in Uganda, but this no longer the case. You now need to apply for an electronic visa online prior to your trip. Go to https://visas.immigration.go.ug/ to apply. A single-entry visa costs US$50 and a six-month multiple entry visa costs US$100.
If you’re traveling to Rwanda and Kenya on the same trip, then apply for the 90-day multiple-entry East African Tourist Visa, which covers all three countries and costs US$100.