Islam means submission to God. Islam is the belief that there is only One God, whose proper name is Allah, which means the God. Islam is the same message given to all the prophets, from Adam, Noah, Moses, Abraham, Jesus, and finally to the Prophet Muhammad, the last messenger (peace and blessings be upon them). They all brought the same message: worship only God, and stop worshipping human beings and their ideas.
Allah is the name of God in arabic, arab christians use the word Allah.
Become a Muslim Now :
If you believe there is only One God who should be worshipped, and no one/nothing else has that right but Him, and you believe Muahmmad, peace be upon him, was a messenger who brought the same message as all the prophets before him, then you are basically a Muslim.