God gave my wife dreams about us moving here to Colorado to join the Ministry. I was going to school to become a computer programmer, and about 2 years into my schooling, God started putting it on my heart (actually he did it much earlier but I ignored it) to join the Ministry. My first thought was, "Pastors don't make any money," and so I dismissed the idea, and went to school for something that made alot more money. Long story short, I chose to join the ministry, rather than do computer programming. As with you, God has given me a rough idea of what to expect, but I do know which position in the 5 fold ministry he has called my wife and I to be in (Missionary). Like others have said, God will reveal things differently to people, but I honest believe that if you are called into the Ministry you will know. It isn't something that just falls into place. It will be something that is in your heart (whether you accept it at first or not), or something you begin to desire, but it will never be by accident. Like you, I felt like I had nothing to offer as well, but that statement alone makes you perfect for the ministry. That way when things begin to happen, you will not be able to take credit for any of it. You won't be able to take credit for your own success in the ministry. I am a horrible public speaker, I stutter, I freeze, I get super nervous, but by the grace of God, I will one day be ministering to hundreds of thousands of people, something I could have never done on my own.
Don;t worry, in time God will show you more of what he has called you to do. Just be patient, and take everything one step at a time. God reveals things to us one step at a time, only because if you are not faithful to complete the first step how will you then be faithful to do step 2-10. My first step was to move to Colorado and study under my current pastor. Hope that makes sense.