Re hourly pay: retail is generally covered (more or less, given the recent workchoices changes) by basic standards: at 15, I wouldn't be expecting too much variation in workplaces: for a 15 yo, should be around $8-9 an hour.
Short of knowing any particular opportunities in Hornsby, I'd go for areas you know -- ie if you're into computers/electronics, try Dick Smith (who I know hire young employees). Similarly, Kmart, Target, etc, tend to be natural potential employers, especially if you've got no past retail experience. All of these have online application procedures (Dick Smith is part of Woolworths, Kmart & Target part of Coles -- both have application procedures on their websites) that you can apply using. Write up a resume, and give it a go.
Other than that, the smaller places might be willing to hire you -- ie fruit shops, etc, (ie FreshCo in Hornsby), who're looking for cheaper employees. Ask to talk to the manager (s/he might be busy, so be prepared to wait a few minutes.. patience is a virtue) and ask if there are any positions available: if not, ask if they would like to hold onto your resume for the future... (But don't necessarily count on them keeping it!)
(Edit: Alternatively, if you have any skills, USE THEM! If you're good with computers, do a letter drop and offer to fix/upgrade/something people's computers. If you play an instrument, do some Busking -- talk to the council and get a permit: I wish I'd done this at your age, because they earn a fortune if done right (Way more than you'll earn in an 8 hour shift in Retail))