Your baby will soon master the art of sitting up unsupported. To help her along the way to this milestone, you can prop her in a sitting position. Make sure she's well supported by something firm but soft — like a semi-circular pillow or bolster — in case she topples. Lay her back down when she fusses or seems to become fatigued.
Give her the opportunity to sit up in different places, and take her on outings in a stroller. The time your baby spends sitting up helps her develop and maintain balance, a key part of her physical development. What's more, making use of the new perspective gleaned from sitting up helps her cognitive development, too. At first, she'll only be able to "frog sit" (with a hunched back and arms forward for support) for a few minutes. Gradually, her balance and strength will improve and before long she'll be sitting up on her own — probably by around 6 or 7 months.

From babycenter.com