The most important thing to remember is- DON'T RUSH IT!! Take it slow. Breastmilk is best, and that is especially true for an atopic child. Just because your baby turned 6 months, doesn't mean you need to rush out and buy baby cereal. Many pediatricians advise delaying solids until 12 months for babies with food allergies. But make sure to talk to your health care provider and do what is best for you and your babe.
I posted a couple charts in the Links section. Both are from Joneja, the author of Dealing with Food Allergies in Babies and Children. One shows which foods should be started at which age (remember, you can always delay longer- these are just guidelines of the soonest you should start each food), and the other shows which foods are most/least allergenic.
Although mainstream doctors recommend starting with rice cereal, it is definitely not the best choice. Grains are the hardest food for a babe to digest, so they should really be started last, and after 1 year when the baby's digestive track is fully matured. (The only reason that rice cereal is recommended as a first food is because it's iron fortified. You can easily find other foods that are high in iron if that is a concern for your baby.) Start with the least allergenic veggies and fruits, then meats.