A. What is fard to know for a boy who has reached puberty is the performance of wudu' and ghusl and what annulls them, the performance of salat and the times when it becomes due, the requirements of fasting and what annuls it, as well as the fact that all that the Prophet brought is true, and the reality of Allah, His angels, His books, His Messengers, the Last Day, the Decree, and the other articles of belief such as reward and punishment, Paradise and Hellfire, the Questioning and the punishment in the Grave etc., and finally what is haram and what is halal concerning acts, speech, food, and clothing. If he marries, or goes to Hajj, or owns property, or engages into trade, then the necessary knowledge that pertains to each of these or other of his situations also becomes obligatory upon him.
What is mustahabb is to learn Qur'an, pray in jama`a in the mosque behind the men, sit with pious shuyukh and people of knowledge, and increase his knowledge of the Religion beyond what is strictly necessary, and to marry according to the Sunna, and other than that.
What is makruh is all that is both unnecessary and does not help in achieving the above. And Allah Almighty knows best.