This is a question that many ask and I'm glad you asked it instead of risking a wrong conclusion. Our classical Jewish teachings explain the "story" which was the setting for this seemingly enigmatic statement.
Moses began to write down the Torah. When he came to this verse - "Let us make man" - he paused since it is in the plural and seems to imply more than one Creator, which he knew beyond doubt was not true. He asked: "Master of the Universe! Why do You furnish a pretext to the heretics to maintain that there is a plurality of divinities?" "Write!" God replied. "Whoever wishes to err will err. Instead, let them learn from their Creator Who created all, yet when he came to create man He took counsel with the ministering angels."
So we are taught the answer to your question. The verse is worded in the plural in order to teach us an important lesson. Humility.
We should never be embarrassed or shy and refrain from seeking the counsel and advice of others. Especially when making an important decision. God is all-knowing and didn't need any advice "sought counsel" - so to speak - with the administering angels that He created in order that we sure learn this lesson of humility and not refrain from seeking advice from others.
I hope this is clear and helpful.
