Ditto to what Dr Ethan has mentioned - bulk building does require a conscientious effort.
Other than avoiding the diet and progressive weight training that has been mentioned, you might want to read up on the various types of weight training routines you could partake in; lower weight, high repetition routines combined with cardiovascular exercise during the same session optimises your workouts.
Strengthening core muscles promotes stability and good physique overall, just ensure you perform them with good form to keep any unnecessary musculoskeletal injuries at bay.
Most of all, enjoy your workouts and stay healthy!
Best,
Shane
