Hi!
As what Ling has mentioned, 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.
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!
Ling Fernandez