Administering First Aid depends on your knowledge, skills and experience.
The key in First Aid is to be aware of what your capabilities are and when to get help.
In the football context, First Aid varies from treating a simple abrasion, to treating an injury, to responding to a collapse.
The best way to obtain First Aid skills is to do a First Aid course. There are a number of these in Australia including those accredited by St John Ambulance, Sports Medicine Australia and the Australian Red Cross.