All foreign visitors to Ethiopia, with the exception of Djibouti and Kenyan nationals, must obtain a visa.
For citizens of most countries it is most straightforward to obtain a visa on arrival at Bole International Airport for a fee of USD 60 for 30 days and USD 150 for 90 days. Make sure your passport has 2 empty pages and is valid for at least 6 months when leaving Ethiopia. Photographs are not needed.
Otherwise E-visa service is available online. A list of countries eligible for a visa on arrival is available on this website.