In Matthew chapter 6, in what is often referred to as the Lord's Prayer, Jesus gave us a model—a model prayer to guide us in how to pray in almost any and every situation. One of the phrases He told us to pray was “Your will be done, on earth as it is in heaven.” In that phrase we find one of the great purposes of prayer.

One of the purposes of prayer is for me to bring my will into submission to God's will. How many times have you had something that was troubling you, and you've gone to God in prayer? You begin to pour out your heart to God. You begin to tell God how you want Him to intervene and what you want Him to do. You instruct Him exactly what needs to happen. But the more and more you pray about it you begin to realize that you need to submit to whatever it is that God wants to do. So you pray, “Lord, not my will, but Your will be done”. So one of the great purposes of prayer is to bring your will into submission to the Father’s will.
 
Once we have surrendered our will to His will, then we begin to ask Him to accomplish His will. As you pray for His will to be done, He works through your prayers to bring about His will on earth just as it is in heaven.

Now there are times that we know the will of God—we know exactly what He wants done because He's revealed it in His Word. In those times we can pray with confidence (1 John 5:14-15). For example, maybe we're praying for somebody to come to faith in Christ. We know that the Bible says God is not willing that any should perish, but all come to repentance (2 Peter 3:9). So when you're praying for a lost person, you can pray, “God, may Your will be done—save this soul in the name of Jesus for the glory of Jesus.” You can know that as you are praying, at that very moment God is working to bring about His will on earth just as it is in heaven.

But there are times that you don't know God's will about a situation. In those situations you pray, “Father, I don't know how to pray. Show me what Your will is so I might know what to pray for.” If He shows you His will, then you can begin to ask Him for His will to be done. He will begin to bring His will to pass on earth as it is in heaven through your prayers.

But in those times that you just don't know what to pray for, you say, “Father, I don't know how to pray, but may Your will be done.” The Bible tells us in Romans 8:26-27 that the Spirit of God intercedes for us according to the will of God. In those times you ask in faith by praying something like, “Father, I don’t know what your will is but I know the Holy Spirit is praying right now according to your will. So I am asking that your will be done in this matter and I want to thank you that you are bringing about your good and perfect will.”

So pray according to God's will, and when you do, you can know that as you are praying, His will is being done and being brought to pass on earth as it is in heaven.