Why is prayer necessary for Christians?
Because it is the chief part of thankfulness which God requires of us; and also, because God will give His grace and Holy Spirit to those only, who with sincere desires continually ask them of Him, and are thankful for them.

What are the requisites of that prayer which is acceptable to God and which He will hear?
First, that we from the heart pray to the one true God only, who hath manifested Himself in His Word, for all things He hath commanded us to ask of Him; secondly, that we rightly and thoroughly know our need and misery, that so we may deeply humble ourselves in the presence of His divine majesty; thirdly, that we be fully persuaded that He, notwithstanding that we are unworthy of it, will, for the sake of Christ our Lord, certainly hear our prayer, as He has promised us in His Word.

What hath God commanded us to ask of Him?
All things necessary for soul and body, which Christ our Lord has comprised in that prayer He Himself has taught us.