Suggest you read the appeals section:viewtopic.php?f=35&t=35032
You cannot normally appeal until March, at which time you ought to be given details about how to proceed.
As am appeal is for a place at a school which has turned down your application for a place (in the normal admissions round, this is by submitting your LA's CAF naming the school as one of your preferences, you won't need to submit the appeal for until you have actually been rejected on March 1st.
Details regarding the appeals process should be in your local authority's Secondary School Admissions booklet - e.g. for Essex CC for 2016 / 2017, it is on pages 18 - 19, so I would assume it to be in about the same place in the 2017 / 2018 version, but I haven't got a copy of that.. It should also be on the website.