Planks, sit-ups, and mostly go to the gym and do all of the core workouts. I also recommend the exercise where you lay on the ground flat, raise your feet 6 inches, and hold them there.
To bulk up best way to train is faster stronger Low fast reps with increasing weight Start out with 20lb for 5 reps move to 40 for 5 reps move to 80 for 5 reps then to 100 for 5 reps to start off that would be one actual set do 10 sets a day Then once you get comfortable with doing those increase the weight or the number of sets To cut you want to do more reps as in 30-40 reps with 40-50lb for 5 sets and move on
I wouldnt suggest only doing low rep and high weight tho, repition is key for tone so there's a middle ground to find in your routine EDIT: Depending on your goals
Key to losing weight is creating a calorie deficit, basically taking in less food than your body needs to maintain your current size. Find your maintenance calories for your size and weight and aim for 300-500 under that daily. Safest way to do so is to aim for 6-8 smaller protein based meals a day with carb consumption focused exclusively around your workout period. Food should be non-processed with no refined sugars. For example, chicken and brown rice is an easy go-to. Throw in as much green veggies as you want to help curve hunger. Drink only water. It's nearly impossible to gain muscle while on a legit deficit, however I would recommend shorter gym sessions focused around the core compound lifts. Squat, deadlift, bench press and military press. 5 sets pyramiding reps 15,12,10,8,6. Going as heavy as you can without breaking form. Throw in 2-3 auxiliary movements of your choice if you feel like that's not enough. Steady state cardio should be done 3-4 times s week either right in the morning on an empty stomach or right after workouts. Shoot for 35-45mins. Follow that and you'll get shredded in no time