What is the First Thing to Call to?

Question :

If someone wants to invite another, how should he start and what should he say?

Answer:

As the questioner seems to want to invite to Allah, inviting to Allâh must be done with wisdom, good speech, and gentleness. He should always begin with what is most important. The Prophet used to advise those messengers of his that he would send abroad, and he used to order them to begin with what is most important. He said to Mu'ath when he dispatched him to Yemen:
"Let the first thing you call them to be the testimony that there is none worthy of worship except Allah and that Muhammad is the Messenger of Allah. If they answer this call, then let them know that Allah has made the five prayers compulsory upon them in a day and a night. If they answer to this call of yours, then let them know that Allah has made charity obligatory upon them; it is taken from the rich from among them and to be distributed among their poor."

So he begins with the most important thing, then the next in importance. He should look for the right opportunity to convey his message in terms of place and time. The situation might call for him to invite the person to his house and then talk to him there; at other times he may deem it more propitious to go to the other person's house and speak to him there. At any rate, though, the insightful and wise Muslim knows how to act when calling other people to the truth.

Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 8 Page 49