K.M.
We started my daughter on "solids"- rice cereal, when she was 6 months. The rice cereal doesn't really provide much nutritional value, though. It's simply to get babies used to eating off a spoon, trying diff textures, etc.
Introducing solids isn't really even necessary until they are about 1 year old, but it's perfectly OK to start around 6 months. Any sooner than that can cause problems with their stomachs since they are so small & their digestive systems are diff than ours.
Before starting any kind of food, I'd wait it out to see if your daughter is just going through a growth spurt. If that's the case, her desire for more formula will slow down.