You can never have the physique you want without first building a solid foundation.
Like a contractor building a house, you must construct your creation from the ground up. I’m sure you couldn’t imagine beginning construction on a house without first having set a foundation in level ground. Neither should you consider going about the task of building your body without a strong base over which to add muscle.
What this means for beginning trainers is to first concentrate on exercises that build strength throughout several muscles, rather than focusing too much on any one muscle. In other words, stick with mostly compound movements, such as squats, rows and presses, in the beginning, then, after you have developed some strength throughout your body, work on details.