In brief, the meaning of “No one is God except Allah” is that nothing deserves to be worshiped except Allah, Who has no beginning and no end to His existence. He is attributed with all proper perfection. All His Attributes are not like the attributes of the creation. Allah’s Attributes have no beginning and no end, and they do not change.
God created all places and He is not in need of any places. God exists eternally before creating all places without a place and after creating places, God does not change. He exists without a place. He is not in the sky or on earth or on the throne. What is in a place has a volume and Allah is not a volume. He created all bodies and He is not a body.
Allah knows about everything. He sees without eyes and hears without ears. He does not need instruments. He is not like His creation in any way. He does whatever He wills, i.e., whatever Allah willed to be shall be and whatever Allah did not will to be shall not be. The Will of Allah as all of His Attributes do not change.
Believing in Prophet Muhammad entails believing that he is the last and the best of all the Messengers of Allah. He was truthful in everything he conveyed from Allah.
All Prophets called to the Religion of Islam. Prophets worshiped God even before they received the Revelation. They never commit blasphemy (kufr) such as worshiping idols and they never commit enormous sins such as fornication. They are also protected from committing small mean sins such as stealing even one grape.