Calling?

Being raised Church or Christ, I am not really sure what to call the desire in my heart & I really don't know what it is. Some would say it is a calling. I am not sure what to call it. Some of my mentors, family, and friends feel that I should be a speaker/teacher. Not just a 7th grade TX History teacher, but an advocate for Partners 4 Africa and a women's speaker. I feel very intimidated at the thought of it all, but at the same time, so drawn to it.
I do not know all there is to know about life or Christianity, but I do have a desire to search for it. Many people along with myself, would look at my life and say, "Penni, you are a sinner. You are the biggest screw up and so what makes you think you can 'teach' me how to be a better Christian? How can you stand before anyone and tell them how to serve God?" Trust me...I think about that every time the thought even passes through my mind. I know that I am not perfect, I know that I am a sinner, I know that I have a long way to go to being where God wants me to be, but I think the important thing is that I know that.
I also know that I love Africa and the people that I have come in contact with there. I want to do all I can here in the states to help them by collecting books, clothes, and blankets. I truly believe that God saved me by making me away to go to Africa 3 years ago. I was headed down a path of destruction, denial, and anger. When I was asked to go...I even said, "Ummm...I can't do this. I am not a good person." But God said you are going whether you want to or not and I thank Him everyday that he sent me. The people there touched my heart. I think about them daily. Serving God and serving those orphans and those who have never known true love or God's peace is what I love doing. But I am here to tell you right now...its not easy, it is most definitely out of my comfort zone, but afterwards it is the most amazing feeling. And if speaking to classes, school groups, and women's groups is what I must do to help them, I will do it.
But many times, I feel intimidated, like I am lacking, like there is nothing I can do to help. I still mess up and sin on a daily basis and so I always have Satan in my head telling me that I am worthless and that I am a hindrance. There are so many times where I am not sure where to start or who to turn to for support.
I understand that I am a woman in the Church of Christ, which means I am limited with out a male in my life...which I find very sad and difficult at times to accept. I know with all my heart that I could do so much good by being a speaker/teacher. I could get schools and groups to donate toiletries and text books, the women's groups that I spoke to could take the information back to their churches and homes. God's works would be spreading and the people in Africa would be helped and blessed.
But how do you know when it is a 'calling' or when it is a selfish desire? I will continue praying and I hope you all will do the same. Or if you have a group that is interested in hearing about Africa and the work that God and Partners 4 Africa is doing...find me and I would love to share my heart with you.