A baby should be able to sit on his own between six to eight months old. If your baby is not able to sit up by this age try and teach him by using a bobby pillow or crossing your legs and letting him sit there for a while and he will get better as time goes by.
Kathbug is right on. Another good way to help baby sit on their own is to sit them on their bottom and spread their legs in a wide V to give them the most stability. But be careful not to push it because of they are not ready to sit on their own their natural instinct is to push backwards so make sure you are either sitting behind them or have a soft landing zone for their little head