I have often wondered about this, and I have a friend who is in deep trouble who tells me that knowing she has a purpose is the one thing that keeps her going.
God does tell us His purpose for us, as one poster has pointed out. Everything we do, every part of our life is pointed to that God wants life for us, eternal life, and abundant life here.
I think that how we live in this life is a matter of our listening to God. It isn't God personally directing our every move, although I think God is a personal God who is part of the direction our life takes. As we submit to Him, as we learn about Him, as we make Him the center of our life there are ways opened up for us to serve Him. I think our only part in this is to take advantage of opportunities that open up to us with faith in ourselves and God, and to spend our time as He directs. In praise of Him, in prayer, in following Him.
The Lord's prayer says it all for allowing God's direction toward our purpose. We are to address the Father, and praise Him. We are to keep within the Kingdom of God and His will in all we think and do. We are to ask for His blessing in all our life. Christ will wipe us absolutely clean each day, we are to acknowledge that and see to we give to others the forgiveness we ask for. And in all, acknowledge that God is over all.
Then, like the guy who jumps into the wind when he is hang gliding, you let yourself be led by the might that is God with trust and faith.