Since you are hoping to play in church it would be a good idea to have the band put on the weekly prayer list, becasue, like Godshouse777 says, prayer is essential.
I've been leading worship at church for a few years so I've got some advice that you may find helpful, or not as the case may be.
First, the concept of learning songs off by heart is good for learning how you guys play together, how each of you approaches a song, and so on, but in terms of leading people in worship it has a big flaw... it offers zero flexibility if you discover that your choices are not really appropriate to the mood of the congregation, which will vary from service to service. You need to have a sufficiently open approach to worship that you can adapt according to where the congregation are, and where God is telling you to lead them to.
Don't put each other down in terms of ability, but equally don't tell someone they're doing a really good job if what they're doing isd detracting from the overall feel of the song. You need to make sure that you are open and honest, without being too severe.
It will take time to reach a point where you feel that you can lead worship in a way that will be a pleasing offering to God. Don't rush, and don't feel that you are progressing to slowly. God will work amongst you and will help each of you to reach the point where you are happy to lead worship at church. It's really important to start your practises with a time of prayer, thanking him for the talents he's blessed each of you with, and asking him to help you to use these talents to glorify him.
Once you know what each of your talents are, who is best at leading worship, who is good at choosing the style of song, who is most able to hold the tune, then it's time to start considering leading worship. But as has already been said, make sure that you only enter into it having prayed about it, and having grown together as a music group. Don't forget that when you are leading worship you are helping people to reach a point of intimacy with God. If you focus on that, you'll find it much easier to do the task that God has set before you, using the talents that he has blessed you with.