You should probably just buy infant rice cereal at the store. I understand your interest in using brown rice (that's what we eat, also), but for now, the instant cereal is probably fine. See what options are available at the store, or look on the internet for wider specialty food choices.
My favorite infant cereal is "Earth's Best Organic" brand- they have mixed grain, oatmeal, rice, etc. It is more expensive than Gerber, but they don't eat that much.
Check with your Pediatrician if you are concerned about digestion, but 6-9 months is usually the recommended time to start cereals.