You will cry, A LOT. Be prepared to see death, suffering, violence and sadness and know that in many cases there is nothing you can do about it.
Be prepared for your marks to be way lower than they were in school - everyone in your class is super bright now.
Be prepared to give up your social life to a large extent, not all your friends will understand when you turn down their 10th invitation because you are on call again. Medicine is not just a career, it becomes your life. Wherever you go you will be a doctor, people will need you and ask for your help and expect you to get it right. Don’t do it for the money or the prestige - neither are worth it!