It all depends on the baby. Some babies are ready for baby food when they are around 4 months and others aren't ready until after 6 months. It's important to make sure that your baby is able to sit up, reach for things and grab them with her fingers, and doesn't use a tongue thrust to push everything out of her mouth when you start solids. Also she will show an interest in other people's food around the time she is ready.
As far as weaning a breast fed baby, part of it depends on why you want to wean and your baby. If your baby is used to a bottle every now and then, it would be easiest to start supplementing feedings one at a time and adding another formula feed every few days so you don't get engorged and uncomfortable. If your baby is self weaning then you will just have to pump for awhile and pump a little less each day to avoid being uncomfortable and mastitis.