"I have an orange."
Translation:Saya punya jeruk.
You could use “Saya punya orange”, that would work all the same. The proper way of saying “I have an orange” however, would be “Saya punya Sebuah orange”. The sebuah referencing to the fact that you have a singular orange because in the English sentence it says “an orange”.
You could also use “Saya punya Satu jeruk”, although the proper way to say this sentence would be the first example I have given.