When babies are around 5 or 6 months old, most of them can be sit up with support, either by resting on their hands in front of them or by leaning on pillows or furniture. Then, by 7-9 months, babies can usually sit alone steadily.
To be sitting up is an incredible change for your baby. She will have her hands free to take objects and explore her environment. In this moment, you can play with her placing toys around her but out of her reach. This will focus her attention while she is learning to keep balance helping herself with her arms.
Sitting will mark the beginning of a new stage in the development of your baby.