Learning to sit up starts way before your baby ever sits down without support. It may start with your baby beginning to raise their chest and gain more control. They may push up onto their elbows during tummy time and hold their head steady without support. This happens at around 4 months old.
At around 6 months old, your baby will begin to explore some new patterns. Your baby may lean forward with their arms extended for balance into a sort of tripod position.
Closer to 9 months old, your baby may start getting into a sitting position on their own. They may start sitting without support.