From the time your baby starts solids around 6 months until their first birthday, it's best to progress gradually from small amounts of simple foods once a day towards a consistent three "complete" meals daily plus snacks. The table below shows roughly how this progression looks for many families, but this is just a guide! Ask your pediatrician for advice and do what's best for you and your family and child.
There is no right answer, but many parents find that early on (~6-8 months) offering solids ~1 hour after a milk feed works well because baby is hungry enough to be willing to eat but not overly hungry & frustrated, and it ensures solids don't yet displace too much breastmilk or formula. Over time, increasing that gap can help baby be motivated by hunger to explore a wide variety of foods.
There is no consensus, which is not surprising given the variation across cultures. In France & Italy, the first food may be a simple vegetable puree, while rice porridge is typical in Vietnam & Japan. A common thread across countries and families with healthy eating habits is an emphasis on variety & real foods, so that's likely a better place to focus than on worrying which exact food to start with.