There are plenty of things you can do at church to help out. They have a lot of programs that need people-power to help coordinate, move furniture, make food, usher people, make decorations, etc.
You're never usless to God. He will use you to reach people that no one else can reach. His mission for you is different from any other person's mission. Just pray about it and be on the lookout. He'll provide opportunities all over the place, and they may not necessarily be through church or any other religious organization. There are tons and tons of ways to share God's love.
you don't need to do mission work to feel "un-useless" to God. I'm sure he understands and sees that your family is tight on money. so instead of mopping around and wishing you can do mission work as an act of prasie to God, why not do something at your church instead! I read your question of you wising you can save your friend and bring her to Christ, so I was just wondering, why don't you set up an out reach with your friends at church? if you have a youth leader, you can start off by telling him/her... and if they agree, start planing an out reach! =]