Praise be to Allaah.
It is prescribed to start with ‘aqeedah (belief, tenets of faith), as the Prophet (peace and blessings of Allaah be upon him) and all the Messengers did, and because of the hadeeth of Mu’aah, [in which the Prophet (peace and blessings of Allaah be upon him) said to him, when he sent him to Yemen]:
"You will come to some of the people of the Book. When you reach them, invite them to testify that there is no god except Allaah and that Muhammad is His Messenger. If they obey you in that, then tell them that Allah has enjoined on them five prayers every day and night. If they obey you in that, then tell them that Allah has enjoined on them Sadaqah (i.e. Zakaah) to be taken from the rich among them and given to the poor among them. If they obey you in that, then exercise caution and do not take their best properties (as Zakaah). Fear the invocation (du’aa’) of the oppressed person, for there is no barrier between his invocation and Allaah.”
This is if the people who are being called are kaafirs (non-Muslims). If, however, the people who are being called are Muslims, then they should be taught about the aspects of their religion of which they are ignorant and where they are falling short, so you should start with the most important, then the next most important, and so on.