Parents always want to know when their baby is going to start sitting up. If you read a pediatric textbook, it will say that your baby should sit up at six months of age. Just remember that this is a guideline.
Some babies show an interest in sitting up from four months of age. They continuously try to do sit-ups to get themselves up. They do these little baby scrunches, and they are trying to lift up using their abdominal muscles.
Some babies need more assistance, and that's okay. You can lift your baby, and hold your baby on your knee, or you can put your baby into baby seat; just make sure it has good support, either for the lower lumbar spine or with a harness like in a stroller, or in a car seat, or in a high chair.
For the babies who don't sit up on time, or are not showing interest, it's good to also give them tummy time, because then those babies often go from the tummy, and then they push up and get themselves into a sitting position or a tripod position.
If your baby is not showing interest in sitting up or not maintaining a sitting-up posture by seven months of age, I would discuss it with your pediatrician.