When I was your age I had quite a few jobs in the service sector, although not food. I do think that the important aspect of working in a customer facing role is, ironically, customer service. There have been so many times I have gone into a fast food place and either not been greeted or served by someone who seemed like they did not care. I do understand that jobs in these places are not particularly exciting and they can be draining from long hours or annoying customers. But to me I at least expect someone working in the service industries to be engaged and friendly. I am not looking for attention to detail or overly helpful, just someone capable of taking the order and getting me my grub in an efficient way.
If I were going for a job in a fast food joint I would tailor my application to show how great I was at giving first class customer service. I would also make nods to being energetic and able to cover shifts at short notice. Places that rely on high staff numbers and generally have a high staff turnaround usually love workers that can come in on their days off and stay a little later if cover is needed. I obtained lots of casual work by explaining I was looking for extra hours and simply wanted to be part of a team. Managers of these places love a committed person who can go the extra mile.
It may be a little white lie but sometimes with huge companies explaining you are looking for a career or least looking to expand your skills can help you stand out. Saying you are just looking for cash or way to pay bills can come off as being unreliable and just looking for a quick fix. The managers of these places will have plenty of staff who only work a few weeks or months as a stop gap and you want to be seen as being in it for a while. Showing you want to learn and gain experience may help you stand out. Lastly, as others have said asking questions and showing interest will be a big help, coupled with a good attitude and a little smile should see you onto a winner.
On a personal note Binary, I am glad you are thinking forward and trying to get a job. Seeing your personal situation unfold from your posts does have me worried and getting a job, even a McJob, will help build your self esteem. It will also help you have a platform to start building your life and moving on from the horrid situation you have been in recently. Best of luck.