When you first start at the gym, your goal should be to get better at using your body the way it was mean to be used. Your body acts as a single integrated unit and your training should reflect this. The simple way to do this is to focus on full body workouts utilizing a variety of upper and lower body basic compound exercises – such as squats, deadlifts, military press, bench press, chin-ups, pull-ups, push-ups, and goblet squats.
This will teach you how to move under control with external load through a fixed movement pattern, develop full body strength, increase your overall muscle mass, and develop a good foundation for more advanced training in the future. Save the upper body-only workouts, the lower body-only workouts, and isolation exercises for down the road after you develop a good base level of strength.