The body is a system, and the best way to train it is as a system instead of as a series of isolated parts.
The best way to train your body as a system is to use compound lifts.
Compound lifts engage the largest muscle groups in the body, as well as all of the support muscles required for stabilization.
By engaging the body's largest muscles, you initiate a more pronounced hormonal response.
This means that by doing compound lifts that stress the biggest muscles, you can stimulate your body to release more growth hormone.
This additional growth hormone can help you see better results from your entire workout.
When I talk compound lifts, I'm referring to exercises like:

• Squats
• Dead Lifts
• Power Cleans
• Hanging Clean to Press
• Bench Press
• Military Press
• Bent-over Rows
• Pull-ups
• Chin-ups
• Dips

All of these exercises are 'multi-joint" meaning that they require movement of more than one joint at a time to do.
Multi-joint exercises by default use more than one muscle group.
Most of these exercises also work the core (abs and lower back).
Squats, Dead Lifts, Power Cleans, Bent-over Rows, Dips - all of these require core stabilization, so working these provides even more benefit.
A good sample workout of compound lifts would be:

• Monday: Squats, Bench Press, Bent-over Rows, Military Press, Dips
• Wednesday: Dead Lifts, Bench Press, Hanging Clean to Press, Pull-ups, Dips
• Friday: Power Cleans, Bench Press, Military Press, Chin-ups, Dips

Do 5 sets of 6 reps for each exercise.
Start low and go up each set, with the final set being around 80% of your 1 rep max.
Aim for progressive resistance - that means going up in weight each workout.
A workout routine like this, supported by enough calories and protein, will really pack on the muscles.
Good luck and good lifting!