Should a new Muslim be taught the rulings of Sharia all at once or in stages, and should one begin with the fundamentals of belief or the fundamentals of legal rulings?

The caller to Islam begins with the fundamentals of religion first, such as the two Shahadas, then prayer, then zakat (definition), then fasting and Hajj when their time comes. This is in emulation of the Prophet, peace and blessings be upon him, as in the sending of Mu'adh to Yemen. The new revert to Islam is not overwhelmed with all the religious laws at once; rather, they are presented to him gradually, lest he be alienated from the religion. This is part of the wisdom in da'wah (inviting to Islam).