You can find a wealth of information in the Appeals sectionhttp://www.elevenplusexams.co.uk/appeals
For an oversubscription appeal you need to show how that school would be the best school for your child and that the reasons you want that particular school can not be provided elsewhere.
Collate the educational and other benefits that your child would gain from being admitted. You should consider things such as type of school (mixed gender etc), the fact it is a faith school, pastoral reasons and the curriculum/ethos.
Please read through the Q&A's and come back if you have further questions.