Solids are usually introduced at around 6 months. A few babies may really something a bit earlier (4-5 months), and quite a few aren't interested or ready until later (7-9 months, rarely longer.)
When you start solids you can start with anything. While it's been traiditonal to start with rice cereal, there is no medical/nutritional reason to do so. Rice is low in allergy potential, but so are most fruits and veggies and meats. And it isn't any easier to digest. The only benefit is that it's cheap and easy to mix up in small amounts .. and for some babies the blandness is a benefit. (It tastes like the milk it's mixed with.)