When you first start training your dog, be sure to train in a 'Low Level Distraction' (LLD) environment.
'LLD' or Low Level Distraction environments are based on your dogs ability, concentration levels and social skills. Each dog will respond differently in each environment that they are in, but as an example of an LLD environment you may want to simply start training in your living room or kitchen at home. Your dog should be trained to a very high standard in this environment before attempting to train in environments of higher stimuli. For example, if you first try training in your local park you may find that this environment is far too stimulating for your dog and so they become almost if not totally unresponsive to training. This type of environment might be considered a 'MLD' (Medium) or 'HLD' (High).