Let me tell you this story about Moses..
Now, Moses was unsure of his speech and wondered how God could ever use him to speak to Pharaoh when for Moses speaking wasn't his strong suit..in fact many people believe that he had a speech-impediment such as a stutter that made him uncomfortable to speak in front of people, His problem wasn't about not listening to God or doing what he commanded..it was how unsure he was of his own abilities to speak in front of a crowd! There are going to be things that God tells you to do that will sometimes make you question your own abilities just like Moses did, but if God needs you in a certain area in the church such as Teaching a Sunday school class, filling in for children's church, leading praise and worship..etc. then I would encourage you to do it unto The Lord and trust in him, we don't always know what's best for us but thank goodness The Lord does! It may feel like that you cannot handle the task set before you but God is going to help you with that task! God will never give us more than we can handle..in fact the bible says so..
"28 Come to me, all who labor and are heavy laden, and I will give you rest. 29 Take my yoke upon you, and learn from me, for I am gentle and lowly in heart, and you will find rest for your souls. 30 For my yoke is easy, and my burden is light.”
God knows what you're capable of, he wouldn't send you somewhere if he wasn't planning on using you in that place! He wouldn't place you somewhere without a purpose in mind! Trust in God and he will take care of the hard things for you, commit to doing what he has put before you to do and do it with all of your might, with a good attitude! God's going to take care of you and he's going to bless your hands for doing that work for him! Don't stress out over doing something that you've never done before..God has entrusted you will a task to do with it the best that you can, God knows what he's doing by placing you there in that position trust his judgement..He knows what he's doing and if you ever feel overwhelmed then get on your knees before God for guidance and he'll comfort you in that time of need!