When people ask me what exercises they should do to tone up their tummy I think they normally expect me to give them a list of things like crunches, sit ups, leg raises, planks, etc, so when I say 'heavy compound lifts, drink more water, and stop eating so much shit and drinking alcohol' I'm usually met with pretty blank expressions.
Toning up your midriff (as with any part of the body) is down to the magic combination of losing body fat and gaining muscle. Ergo, the best way to 'tone up' is to train for hypertrophy (muscle gain), drink lots of water, and eat a healthy balanced diet...always remember that your results are 80% down to nutrition, you can't out-train a bad diet!
Looking specifically at the fat loss part of the picture (because everyone has a six-pack, just some people are hiding it under a layer of fat), training-wise I would recommend working the whole body with weights (either in each workout or split into different body parts for different days) and if you're just starting out and want to keep it simple just aim to hit a rep range of 8-12 for 3 sets on each exercise. Focus on compound lifts (those using more than one joint) like squats, deadlifts, rows, and presses. This is because they use the biggest muscle groups, which means they burn the most fat as well as helping your body stay balanced in terms of muscle (not to mention all the other benefits, but stay tuned for another article on that topic soon...)
Core exercises don't burn much fat, so although you may want to include these at the end of the workout it's not worth making them the focus. For an extra boost try including some high intensity intervals of your favourite (LOL) cardio such as running, rowing, swimming, cycling, etc.
For me, I do core exercises not to 'tone up' but rather to strengthen the muscles to help with my squat and deadlift. So if you REALLY want me to recommend a core exercise then I would go with either hanging leg raises, or ab rollouts (check my Instagram video below).
Get these tips on the go and let me know how you get on!