It is often said on here that you have to show that the appeal school is the only school for your child. That simply isn't true. You have to show that it is better for your child to go to the appeal school than the offered school. The fact that there may be other schools in the area that are even better for your child is irrelevant, even if it is true.
Fundamentally, to win your appeal you need to show that the disadvantage to your son from not attending this school outweighs any issues the school will face from having to cope with an additional pupil. Things like transport difficulties don't win appeals. You need to highlight things this school can offer your son that are missing from the allocated school and are particularly relevant to him. So, for example, if he is musical and the appeal school has more extracurricular musical activities, you should bring that up.
Remember you are appealing for the school you want, not against the school you have been offered. In general it is not a good idea to suggest that the allocated school is unsuitable unless the case is very clearcut (e.g. your child is confined to a wheelchair and the school is not fully accessible).
In terms of process, the letter refusing entry will have told you how to start off the appeal process. You don't need to send in your full written case immediately. Some people submit with just a few bullet points, or even with a simple statement that their written case will be submitted later.
Before the hearing, the panel will read your written case and any evidence you submit. During the hearing you will be asked to present your case. Don't simply read out your written case. That won't help the panel. Use it to highlight the strong points in your case. You also need to be ready to give a short summary at the end of the hearing.
You will receive the case to refuse entry before the hearing. Look for weaknesses that you can raise, either during your spoken case or by questioning the admission authority's representative. If you share their case on here when you get it you will get advice on challenging them.
If you need any information from the school or the LA to help you prepare for the hearing you should ask. They are required to answer any reasonable question you ask.