The Components
The P wave represents atrial depolarization, the QRS complex represents ventricular depolarization, and the T wave represents ventricular repolarization. The PR interval reflects the delay through the AV node, and the ST segment sits between ventricular depolarization and repolarization.
A Simple Reading Approach
Ask five questions in order: is there electrical activity, is the rhythm regular, what is the rate, is there a P wave for every QRS, and is the QRS narrow or wide. This method keeps you from jumping to a conclusion on partial information.
Scope of Practice
EMTs are not expected to interpret complex rhythms, but you should recognize life-threatening patterns such as ventricular fibrillation and ventricular tachycardia. Always treat the patient and not the monitor, and pair the rhythm with the patient's pulse and symptoms.