The giving of the Ten Commandments represented a major transition for עם ישראל. They went from being Noachides plus a few more Mitzvot, to becoming an עם סגולה וגוי קדוש, a nation of priests and a holy nation.
According to the Rambam, the first commandment represented the most fundamental concept of all- the existence of G-d.
When the Torah says, “I am Hashem that took you out of Egypt”, the meaning is that G-d exists.
We are to be aware of Hashem’s Presence and that He is running the world.
This is why many synagogues have the Pasuk, שויתי ה׳ לנגדי תמיד, that Hashem is opposite us at all times. If we are aware of Hashem’s existence at all times, we will be reminded how to behave and act in a way that is pleasing to Him.