Welcome to the journey of learning to hear the voice of God more clearly. If the Lord is asking you to do something, He will make it clear. The prophet Jeremiah received a message from the Lord to buy a field. His cousin came to visit him in prison and told him the land was available for purchase (see Jer. 32:6-8). After the circumstances lined up, Jeremiah knew that the message he had heard was a sure word from the Lord.
If we are not sure if we should walk through an open door that is before us, we can trust God to speak within our circumstances. Two high school students told me they were wondering what the Lord was calling them to. Then they realized God had placed them in school for a reason—to be used exactly where they were. They focused on school in a new way, and began inviting their school friends to church every week. Many of the young people responded. School was the circumstance God used to provide open doors for their ministry. What situation has the Lord placed you in right now that may be the open door He is calling you to walk through?