First of all, don't panic!12 year olds are manageable if that is what scares you...if you're worried about teaching material, try to introduce topics that would appeal to teenagers. It is important to present material that they may consider childish or else they will not stay focused for long. You may want to do a listening with popular songs you think they may like (fill in the gaps)that has always worked for me.The important thing is to get to know your students and the rest will come naturally...
Happy teaching!
Christina Giannikas