If you do not want your parakeets to have babies, you will need to separate the male and female birds. This can be done by housing them in separate cages, or by using a divider in a shared cage. You will also need to remove any nesting materials from the cage, such as twigs, leaves, or shredded paper.
If you are not sure whether or not your parakeets are sexually mature, you can consult with a veterinarian. A veterinarian can also help you to determine the sex of your parakeets, if you are not already sure.