When the word next to a/an starts with a vowel (A,E,I,O,U) we use “an”, however when the word next to a/an starts with a consonant (B, C, D, F, G, H, J, K, L, M, N, P, Q, R, S, T, V, W, X, Y, Z) we us “a”
a book
a pen
a house
an engineer
an umbrella
a beautiful car
an apple
a mango
a doctor
a cow
a buffalo
an orange