Suggest taking a Level 1 sports physio course if there's any about. There are a lot of basics you need to know, especially formulating an onfield assessment. The old TOTAPS approach (Touch, Observe, Talk, Active, Movement, Passive Movement, Skill Test).

The Physio Shop has a few useful pocket books for this. Here's a few ideas

Pocket Book of Taping
Field Guide To Fracture Management
The 5-Minute Sports Medicine Consult
Sports Emergencies - Management Scenarios
Atlas of Living & Surface Anatomy for Sports Medicine with DVD