If you have a single horn, it's fingers 1 and 2 for A natural, and fingers 2 and 3 for A flat. If you have a double horn, you can add the thumb key to open the B flat side of the horn.
The key would be Ab for horns, 4 flats in the key signature, so starting on low A, it goes Ab (2 3), Bb (1), C (open), Db (1 2), Eb (2), F (1), G (open), Ab (2 3). The fingerings go along with a...