"He reads his book."
Translation:Han läser sin bok.
That's the problem – you basically need to learn that along with the word. There are some tendencies, but no very good rules. (for instance, most living beings are en words, but there are many exceptions too, like ett barn 'a child'). So your best bet is to try to learn the gender when you learn the word.
Oh and most nouns (like 80%) are en gender, that's also useful to know.