If you're serious about breastfeeding think of it as the only option initially. If you think as formula as an easy out and have a can on standby you're more likely to fail. If you're having difficulty breastfeeding there are lactation consultants that will help you. You can find one by asking your doctor or midwife or looking in the phone directory. It's common for babies to have a sensitivity to some sort of protein in the mom's diet - usually the big ones are dairy and soy and nuts. If you want to educate yourself more regarding breastfeeding, Breastfeeding.com - Everything for the New Mom is a good resource. They have a forum to ask people questions. Also La Leche League is a wealth of knowledge.
To feed a baby it's best to feed what was designed to feed them - breastmilk or donor breastmilk. If that doesn't work then formula is the next best option.