CPR is a vital skill to learn and it is a part of Emergency First Aid Training Courses, like the ones from https://www.tidaltraining.co.uk/emergency-first-aid-training-courses. During the course they will support you in learning a number of skills that will help people who may have had an accident or injury and also support those suffering from life threatening issues such as a heart attack or cardiac arrest.
CPR should only be conducted if the person is unconscious and either not breathing properly or not breathing at all. When you first come across a person who is unconscious you should shake them gently and speak to them loudly to try and gain their attention. If you get no attention you should call 999 and inform them of the situation. They will talk you through CPR and direct you to a local defibrillator if there is one nearby.
The steps of CPR are to give chest compressions by placing one hand in the centre of the chest, the other hand onto and interlocking the fingers. You should straighten your arms and use the heel of the hand to push down firmly. You will need to push down about 5cms. You should be doing a chest compression around every 2 seconds.
It can be very tiring doing CPR on someone, so if there are other people around that can assist you, that can help alleviate some of the stress and tiredness that starts to set in.