When God called Moses into the ministry (Exodus 3-4), Moses resisted the call because he didn't feel he could do what God wanted him to do. He asked, "Who am I that I should go to Pharaoh, and that I should bring the children of Israel out of Egypt?" (Exodus 3:11). Moses argued that he was "slow of speech and slow of tongue"(Exodus 4:10), and God reminded Moses that He had made his tongue and could easily teach him what to say!
Such resistance or hesitancy to answer a call is normal. After all, it's an awesome thing to be God's servant and to do His will.
Now, it's one thing to resist God's call and quite another to refuse it. That's what Jonah did, and he paid a price for it.
To be an effective servant of God, remember these three truths:
#1 - If God has called you, He hasn't made a mistake. He knows what He's doing, even if you don't. The best thing you can do is gratefully submit to His will and trust Him to do His work. God prepares those who He calls, and you will always be a work-in-progress. God sees something in you (that you don't see in yourself) and He knows what you can become and what you are able to accomplish. Like Gideon, He can turn a nobody into a somebody.
#2 - If God has called you, just be yourself. One of the biggest mistakes in ministry (of any kind) is that servants try to be somebody they are not. Perhaps they want to imitate one of the big boys. Or maybe they allow the expectations of other people to manipulate them into becoming a person they are not. Listen, you are unique just like you are. God has wired you a certain way, and prefers that you serve Him that way. To be sure, we can learn from other people. Additionally, we all still have more growing to do, and this involves change in our character, etc. But God forbid that we should become a facsimile of someone else!
#3 - Build on your strengths, and ask God for helpers to compensate for your weaknesses. Nobody is perfect, and no one person can do everything. Moses had his brother and sister for assistants. His father-in-law became an advisor. Moses appointed captains over different sized groups of people (10's, 50's, 100's, etc) to help with the volume of demands on his time. And you too will need someone to assist you to compensate for the things you can't do.