No need to be nervous. If you've never coached before, I would approach it differently than you may be thinking right now. Ask the owner if you can meet with him/her about your future aspirations for cheer. Tell the owner that you're interested in coaching in the future and want to know if you can begin training as a junior coach and get your feet wet a bit. Can you sit in on practices and classes and observe the coaches? Can you assist with a tiny or mini team?
Once he/she says yes, here are some things you should be sure to do:
1. Take it seriously. Practice isn't the time to cuddle the little ones, play with them, pick them up... They'll never respect you if you act like a babysitter instead of a coach. Break time is the perfect time to bond with the little ones. Just show them you mean business when practice is in session.
2. Ask questions. While you're getting comfortable with instructing, ask your coach for confirmation that what you're saying is correct. Avoid stepping on their toes. The first few practices you may want to ask permission to address the team with a correction or thought. You don't really need it, but it shows that you respect the opinion of your coach and that you understand that you don't run the team.
3. Don't be afraid to step up. After you've taken a few practices to get comfortable, jump right in! Don't contradict your coach, but be hands on and show the kids and coaches that you are confident in your coaching abilities.
4. Learn the back side of the sport. Sometimes coaches forget that there's more than just practice. Take the time to study scoresheets and rubrics. Learn the USASF rules for your level inside and out, and have a basic understanding of the other levels as well.