As soon as your baby can hold their head up without support (usually around three or four months), you can start sitting them up in a supported position such as with their back against your body. Most babies like this as it gives them a clearer perspective of their environment.
The more often you do this, the sooner their muscles will develop. When your baby is around six months, they will likely start to sit more independently. If they haven’t started sitting on their own by eight months, you may want to mention it to their pediatrician (source).