Well first of all, Mr or Ms mesomorph, just count your blessings to have been born with the ideal body shape.
As a man you are naturally wide shouldered and slim hipped.
As a woman you are a classic hourglass, whether fat or fit.
Any sound diet you undertake, any solid exercise routine you do (whether cardio or resistance training) will have positive results for you.
You will achieve the size and shape you are after much more quickly than the rest of us mere endomorphs and ectomorphs.
The risk for the mighty mesomorph is to take your somatotype (body shape) too much for granted.
Even when you are out of condition and carrying too much body fat for your health, you tend to look pretty darned good (unless you've let yourself go completely).
You have to look after your body because one day, what you put into it will reflect in your health and appearance.
You will pay dearly for complacency, so don't go throwing away your natural advantages.
With regards to diet, choose a nutritious balance of mainly whole and unprocessed foods.
Moderate to high carb intake and moderate protein intake.
Minimize simple carbs and saturated fats.
Your body thrives on explosive bursts of exercise; the type you get with high intensity interval training (cardio) and supersets in the gym.
You do not need a great deal of rest between sets or exercises, as your body is very strong, thriving on the nutritious food and regular workouts.
While you will do well with any reasonable diet and exercise program that takes your fancy, you really should become as informed as you can about what types of foods suit you best and how to maximize your fitness as a mesomorph; knowledge is indeed power.