For the first four months if you have enough milk and the baby is sucking well and gaining enough weight then that is all the baby will have. If even after four months you have enough milk for the baby you can carry on with only breast milk .
If however you are supplementing your breast milk with a top feed during the first four months; then once you complete four months your doctor will ask you to start the child on its first semi solid food.
The first food will be apple or banana. Start with any one and give only that for the first week to see if the child is accepting it or allergic to it. He is most likely to spit it out the first time but slowly will get used to it.
Once you start semi solids you should have separate vessels for the child. The cooker for the apple should also be a new one and should be washed carefully. Apple needs to be stewed and strained or pureed before the child eats it. You do not need to add anything to it.
In the case of banana simply mash it with a fork and you can add some of your breast milk to make it watery. Do not add sugar to this as well. Once the child eats both fruits you can alternate both fruits every day before you start on his next semi solid. It is best to give the fruit in the morning or in the evening so that the pattern is set. You do not need to feed your child breast milk after that. You would however need to start giving him sips of boiled water.
Once the baby is eating these fruits you can on your own after a month introduce some new fruits as well. Encourage the child to eat healthy as when they grow they have loads of junk food options which will not be in your control.
Once your child starts eating even keep his spoons and forks separate and after washing keep it in some boiled water.