I have three kids, and I started the first one off on rice cereal early on. Four months, I think. My pediatrician advised this and I did as suggested. I was young, and it was before everyone had internet, LOL. When my younger children were born, I waited until they reached six months before introducing solids. I read some research on this topic, and found out that the baby's internal organs really are not ready for solids until later.
Some signs of solid readiness in infants are sitting up independently and having the grip that allows them to pick finger foods up themselves. You can even do self-feeding, but we used a combination of home made baby foods and store bought organic baby jars.
I see Google is already your friend, LOL. Please call on your friend and type in phrases like "infant solids research", "infant gut maturity solids study", "reasons to delay solids until six months", "rice cereal as a first food", and so on.
By the way, I think mashed bananas are great. We also used sweet potatoes, avocado, apple sauce, and many others. You may want to wait with things like strawberries, to which many babies are allergic, until your baby is a year old.
I hope my ramblings are of some use to you. I'd be happy to answer any other questions you may have.