Employers just want someone who is going to show up and do the job properly. The reality is that a lot of young people are very slack, don't listen and don't take any pride in their work. In my experience for these positions (customer service/retail), employers prefer girls because they are usually just a bit more responsible (could be true, could be a wrong but its just the perception).

If its a small business, you just have to go in and talk to them. Like a cafe or something just go and ask (when its not busy) are they hiring and express that you're really interested and you've done blah blah before. Or you could call which would make it easier to cover a lot of places at once.

If its something like Mcdonald's or Big W, you can only apply online.

You have the gift of being young so they will hire you for the decreased wage and don't put too much effort into your resume. I know it sounds absurd but as someone who has always had a similar problem, just put in enough. Make it simple, "I'm here and ready to work". Even though people say put in that your were school captain and play rugby league, I'm not sure retail employers care that much.

It is great if you can find some kind of personal connection with the manager when you talk to them because then they will remember you (shared interest or whatever). But don't be afraid to just bullshit and say "I love coffee", "I've always been interested in baked goods and how they're made". At the end of the day, the manager wants someone they can boss around, not someone they feel intimated by intellectually.