Look for whole grain as the first ingredient on the list, which will indicate what the cereal contains the most of. Also, check to see if there's at least 3 grams of fiber for every 100 calories. When it comes to cereal, the more fiber, the better.
Cereal should be low or free of saturated fat. Normally saturated fat isn't found in grains, but some companies add so-called yogurt coating or chocolate that may contain palm or coconut oil in them. So look for no more than 1 gram of saturated fat per serving.
Spin-offs of tried and true cereals don't need the added goodies like chocolate or so-called yogurt clusters, as the new additions are generally adding things you don't need. So stick to those originals.
The recommended daily amount of sugar in a whole day is 40 grams, or about 10 teaspoons of sugar, so you want a lot less in your cereal. You'll want no more than 8 grams of sugar, which is about 2 teaspoons, for the recommended serving size.