The one and only Great Commandment is to love God with all our hearts, minds and souls:
Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. This is the first and great commandment. - Matthew 22:37-38
Concerning prayer,
After this manner therefore pray ye: Our Father which art in heaven, Hallowed be thy name. - Matthew 6:9