"An orange is a fruit."
Translation:संतरा एक फल होता है।
Other reply addressed their gender and plurality, but in case you meant when to use any of them - in the habitual aspect. I.e. something that is generally true, not just true at the moment or for a specific instance.
Note that it's a property of grammar, not of vocabulary or logic, so you can say the same sentence (modulo grammar changes) in the habitual or other aspect, and have it be valid in each case.
The way I understand it, (native speakers please correct if I'm wrong!) is that एक gives much more emphasis to singularity than 'an', it's closer to 'one' even when not used as a number.
So, just as in English, saying 'one orange' is a fruit makes the listener think 'what about the others?'
Especially in this sentence with होता, the combination to me starts to sound like a children's story: 'there was one orange, different to all the others, he was a fruit'!