Mainly on their holy book, the Qur'án.
because it shapes the conduct of a muslim
The belief that Almighty Allah (God) is the Only Supreme Authority makes a Muslim dauntless, honest, and truthful.
In Islamic countries.
Yes, a Muslim girl can marry a Christian man if he converts to Muslim. However, the man convert into Islam should be based on true and deliberate belief in Islam and Islam principles not converting to Islam only to make the marriage legible with the Muslim girl. See related question below for more information.
No.
how muslims belefs in prophets influences the daily of a muslims
There is a mix of Afrians and Arabs, they're muslim.
There is no god but Allah and Muhammad is the messenger of Allah
No. He is Roman Catholic and rather devout in his belief.
NOT NECESSARILY. Belief in Allah is a necessary, but insufficient condition to be a Muslim. The primary statement of belief in Islam is the Shahaada and the Shahaada has two components: (1) There is no god but Allah AND (2) Muhammad is His Messenger. Failure to believe in Muhammad's prophecy means that the person in question is not actually a Muslim.
If you are Muslim man, then yes you can marry her on condition that she is Muslim, Christian, or Jew. If you are not a Muslim man, you can't marry the girl friend if she is Muslim unless you convert; with true belief and free will; to Islam.
It means a country where Muslim is not the founding or dominant religious belief. It doesn't mean there are no Muslims in that country. Nor does it mean that a particular country bans the Muslim religion.