First - get this devotional book and read it every day! Hearing God Through the Year: A 365-Day Devotional, by Dallas Willard.
My definition of prayer, following Willard, is this: Prayer is talking with God about what he and I are doing together. God speaks to me about what he and I are doing together.
What are God and I doing together? We are talking about bringing his kingdom realities on the earth.
"God’s speaking to us does not make us important. Just as when he spoke to the ancient people of Israel, his speaking to us gives us greater opportunity to do good and greater responsibility for the care of others. God speaks to us because he wants us to join him in some purpose of advancing the kingdom of God here on earth." (P. 45)