Typically you agree with someone, you agree about a topic, and you agree to do something. I agree with my husband about politics. I agreed to take my children to a movie if they finished their homework. Unfortunately my children can't agree about what movie to watch.

You "agree with someone". Examples: 1. I agreed with Tom that we should go to Hawaii next year instead of Canada. 2. We agreed with him that she was rather rude to them. 3. He always agrees with his friends on every matter. You "agree about" or "agree on" a particular topic. Examples: 1. We agreed about everything that was mentioned during the meeting. 2. Generals of Ukraine and Russia agreed about ceasefire at the airport of Donetsk. 3. Dan and Stacy agreed about merging both of their classes and teaching together. 4. We agreed about/on which movie to see. 5. They agreed about/on several topics. 6. The children agreed about/on what kind of Christmas decorations to make for Mrs. Taylor. You "agree to do something". Examples: 1. I agreed to pick up her boyfriend from work. 2. She agreed to work tomorrow instead of on Friday. 3. We agreed to stop arguing in front of the children. You "agree that something is true or false." Examples: 1. They agreed that the weather in Hawaii is much better than the weather in Boston. 2. We agreed that the teacher was too strict on the kids. 3. Who also agrees that the ESL program in Hatfield isn't all that great?