T.,
This is a great question to ask.
The American Academy of Pediatrics recommends exclusive breastfeeding for 6 months before introducing complimentary foods. Breastfeeding should continue to a minimum of one year (with 75% or more of nutrition coming from breastmilk at 1 year) and thereafter as long as mututally desired.
The World Health Organization recommends exclusive breastfeeding for 6 months and continued breastfeeding with complimentary foods to 2 years or as long as mutually desired by mom and baby.
Developmental signs of readiness for solids include the following:
1. At least 6 months of age
2. Able to sit up unsupported
3. Developing a pincer grasp
4. Showing an active interest in eating. trying to put food in mouth, mimicking chewing motions, etc.
5. Loss of tongue thrust reflex.
Is there a reason you want to start with cereal? It is not necessary to start with cereal, or even with purees. The wikipedia page on Baby-Led Weaning (offering appropriately sized pieces of table food believeing that if the child has the skills to pick up the food and put it in his or her mouth, they also have the skills to chew and move the food to the back of the mouth for swallowing) is a great explanation. Cereal is a carb loaded filler with little nutritional value aside from iron which is more bioavailable in your breastmilk. Starting solids too early is thought to increase the incidence of food allergies. Check out the information on "open gut" in infants. There's no harm in waiting to introduce solids.
We started our daughter on solids at 6.5 months. She got chunks of banana, avocado, and sweet potato (spaced out appropriately). We have had minimal issues with gagging (which is a normal part of learning to eat) and no problems with choking. At 9.5 months she's eating a pretty good variety of table foods already. It's also a lot easier and less expensive to feed her what we're already eating.
S.