From what I've read, sitting is a developmental milestone that baby will meet typically around 6 months of age. But as with all development, every baby is different and some may be earlier or later. Quite a few of my friends mentioned that their babies were still learning to sit unsupported after 7 months.
You may find this article useful to help encourage your baby to sit on his/her own:
http://day2dayparenting.com/help-baby-sit-up-independently/
Mentioned in the article are some precursor skills that baby has to master before he/she could attempt to sit without assistance. For example, strong neck and back muscles to allow baby to hold his/her head steady, and rolling for good trunk muscles. The article provides a list of useful tips how one could help develop the muscles and positions to help baby pick up sitting.
Hope this helps!
Anonymous