You should know your goals precisely and how you're going to achieve them if you want to get the most out of a personal trainer.
For example, maybe you want to lose weight. Know exactly what your goal weight is so that you can track your progress so that your personal trainer can better help you achieve your goals. Maybe you want to gain muscle, then your personal trainer can personalize your workouts, and meals if desired, to build muscle.
Knowing your goals also helps you to track your progress and keep you working towards your goal without getting discouraged.
You can't outrun a bad diet. If you're not getting the results you want, maybe it's because you need to take a look at the foods you're eating. Your personal trainer is meant to help you achieve your goals, but you need to do most of the work yourself.
To get the most out of a personal trainer, you need to evaluate your goals and adjust your diet accordingly. If you want to lose weight, you need to eat at a calorie deficit. If you want to build muscle, you should be incorporating more protein into your diet.
If you eat right and stay on track, you will be getting maximum efficacy out of your personal trainer and your hard work will start to pay off.