"Eleni eats the apple."
Translation:Η Ελένη τρώει το μήλο.
This business with proper nouns needing the article in front of them, and there being masculine/feminine/neuter forms of articles, is so commonly asked (I searched high and low for an explanation because it's not at all intuitive to a native English speaker) that I hope they add a "tip" about it to the Basics 1 lessons at some point. It would really help!
Simple: The article "o": masculine, how do you know were to put it? Just look at the end of the next word. If it ends with an "s" (and it is not in plural), then it's definitely a masculine. (ο Γιώργος, ο καναπές, ο ελέφαντας, ο καθρέφτης)
The article "η": feminine If the end of the noun is "η" or "α" (again not in plural), then it's feminine. (η Γεωργία, η ομπρέλα, η στάση, η τσάντα)
The article "το": neutral. The neutral nouns end with "ι" or "o" or "υ" (το τραπέζι, το βιβλίο, το παντελόνι, το κορίτσι, το δάκρυ)
Sometimes neutral nouns ends also with "α" (το πάπλωμα, το κάλυμμα), but they are not so many.
You don't, Greek orthography is historical so there's not always an explanation. There are some patterns though which you will pick up as you progress further down the course. About the apostrophe, it's the second button to the right of the "L" button on your keyboard, I guess.