The standard mainstream advice is to start with baby cereal at six months. The argument is that the iron is necessary for babies. This was true thirty years go and more, when scientists hadn't yet figured out how to get absorbable iron into formula, and babies were only very rarely breastfed in this country. It's really not true anymore. Breastmilk has less iron, but it is in an highly absorbable form. Other than the iron, rice cereal is mostly just starchy filler with some synthetic vitamins added. And there's evidence that grain foods fed too early are not healthy for the gut.
It is true that it is safe to introduce solid foods around six months, but many babies wait even longer, and that's fine and probably best. And once you do begin, fruits, vegetables, and foods high in healthy fats and protein are healthier options.
Our favorite first foods: ripe banana mashed with some breastmilk, or chunks of very soft ripe banana that baby can pick up, applesauce, ripe avocado, egg yolk, baked pears spiced with cinnamon, plain unsweetened yogurt, sweet potatoes.
If you really must use cereal, try to find a whole-grain version, or just puree fresh cooked rice and mix it with breastmilk. Less processed.