Have a quick chat, and then stand in the stream of light. Open the inventory and look under "books." Use the Elder Scroll. Watch the scene from the past.
After your vision, you will have learned Dragonrend and can use it. At this point, Alduin will attack and you will have to fight him. Watch out for the falling meteorites while looking up. Try to hit him with Dragonrend which will force him to land. In the case of Alduin, he MUST be under the effects of Dragonrend to deal any damage to him. He will glow a blackish-blue when he is affected. It is possible to keep him grounded once he lands because the shout lasts for longer than it takes to recharge. If this works to your benefit, use it. In this case, since there is so little cover (can try using the word wall?) I recommend letting him take off so you can heal and then landing him again. Keep going until you empty his health and make sure you’re aiming for him and not Paarthunax who is helping you.