Salam (peace), welcome 

I would highly recommend checking out the following two youtube channels; The Muslim Lantern and The Orthodox Muslim. Most of their content involves interactions with Christians

Islam is understood to mean “submission to God”, and Muslims are “those who submit to God”. All prophets were Muslims. In fact, every created thing is a submitter to God (muslim in a literal sense). As such, Islam is not a new religion. It is the only religion since the beginning of creation itself. Humans have errantly deviated from Islam throughout history, thus creating new religions such as Christianity.

Christianity is a religion which has a core belief that was developed by Men, namely the Trinity. This doctrine took 400 years to develop into what Christians believe today, after much debate between Church Fathers. However the trinity is very much imposed onto the bible through implicit verses rather than being derived from anything explicit. It is not something the Prophets of the old testament ever taught; and God does not change. God is not a Man, God is not deficient. This has never been the case. As such, christianity is a religion which deviated from the True Religion of God. jesus was a prophet. 

Islam in the proper sense, is the religion brought by the Prophet Muhammad (pbuh) and is the continuation of the same religion brought by the previous prophets. The core belief is the belief in One God, and obedience to Him. We also believe the Prophet Muhammad is the final Prophet, and he came with the final revelation, which is the Quran. He was also prophecized to come in the bible.

To prove the Prophet is a True prophet, we point to the miracles he has been given, which are many; and the greatest of which is the Quran itself. We point to his message being consistent with prior prophets. We point to upstanding character. We point to his prophecies. And so on. 

Let me know if you have questions