How can I know God's Will?

The question of how to know God's will is one of the major concerns of a Christian.  Particularly for 20 somethings who want to know what to study in college and whom to marry!! All of us wrestle with wondering if this or that is what God wills for us.  How can we know? Do we just tack "if it's your will" at the end of our prayers to be "safe"?  If He is sovereign and accomplishing His purpose for the world, why even pray? If I just believe hard enough, can I have whatever I want? Is that what the passage in Matthew 21:2 means?

  And whatever you ask in prayer, you will receive, if you have faith

A misunderstanding or misapplication of this verse has led to much heartbreak, much self-condemnation, even some abandonment of the faith.  Surely we have all prayed for some particular something and then it didn't happen as we asked.  If we don't know what this Truth is, we are left with anger at God or at ourselves.  Either He lied to us or our faith was inadequate. 

Neither of these is correct.

When studying Scripture, know that the best commentary on Scripture is Scripture itself.  In other words, look at other passages on the topic to get the explanation.  Consider the whole counsel of God, not an isolated verse. 

So to gain understanding of this passage, we look at
   Hebrews 6:18  so that by two unchangeable things, in which it is impossible for God to lie, we who have fled for refuge might have strong encouragement to hold fast to the hope set before us.

and Numbers 23:19 God is not a man, that he should lie, nor a son of man, that he should change his mind. Does he speak and then not act? Does he promise and not fulfill?      

God does not lie.  Period.  Then, is my faith inadequate?  If I had believed "better", would my request have been granted?

No, that's not what it means.  And we can know this because of Matthew 17:20 which says
For truly, I say to you, if you have faith like a grain of mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move, and nothing will be impossible for you

Ok, then, how do we get our prayers answered? First, look at what the Bible says here:
And this is the confidence that we have toward him, that if we ask anything according to his will he hears us. 15 And if we know that he hears us in whatever we ask, we know that we have the requests that we have asked of him  I John 14:14,15

His Will.  There it is again. His Will.  When we pray in accordance to His Will, He grants what we ask.  A couple of questions come to mind - 1)What's His will?  The answer is in John 15:7 If you abide in me, and my words abide in you, ask whatever you wish, and it will be done for you.

God's Will is His Word.  When we pray His Word, we are asking according to His will.  For example - when I pray for my husband, I ask God to cause him to "walk in a manner worthy of His calling" (Ephesians 4:1) or for my children to "love God with their whole heart" (Mark 12:30).  In fact, I have a whole set of verses that I pray regularly for myself and my family.  Even for specific things like breaking down strongholds of pride or despair or anger (2 Corinthians 10:3-5).  Praying God's Word is praying His Will.  We are assured that His Word is His Will for us so we can pray with confidence that He will grant what we ask.

2)What about things that are not precisely addressed in Scripture, such as whom to marry, what to major in, where to live, etc?  Well, many of these concerns are addressed in principle and God calls us to search His Word, seeking His direction.  But sometimes, not.  In those instances, I pour out my heart and tell God my desires.  I ask for His guidance about what to ask, how to pray.  And, then I submit to His Will.  I like how Henry Blackaby puts it "And, Lord, if you have something better in mind than what I'm asking, then cancel my request!"
As I have practiced prayer longer, I have learned to "listen" with my heart and often the Holy Spirit directs me how to pray.  Sometimes I don't hear correctly, to be sure, but what a delight when I do.  It is so exciting to be led in prayer to ask God for something and then see Him give me what I asked for.  Remember the story of Chip and the Indian sheets? God Himself puts His desires in our hearts, so that He alone can fulfill them.  What a faith-strengthener!!!

And, remember, prayer is God's means for getting His Will done on earth...for changing us, conforming us to His image.  Learning to listen to Him and learning to want what He wants.

Sometimes we think God hasn't answered.  But He does.  He often says "yes" and we see the answer and we rejoice.  Other times, He says "no".  And it's perfectly OK to be disappointed or sad for a time.  Tell Him!!!  Pour your heart out to The One who can do something about it.  The One who can bind up your broken heart.  But sometimes, He just says "wait".  Those can be very difficult times.  We can wait on Him a long long time and we can be tempted to think He has said no.  Or nothing.  But don't give up.  Please don't give up.  He is good.  He gives good gifts.  And His plans (His Will) for you are good.