A baby can be put in a high chair as soon as the baby is able to sit up on it's own without assistance and is eating solid foods then the baby is ready to sit in a high chair.
A baby is usually ready and able to sit in a high chair from 4 to 6 months of age.
Also always strap your baby into the high chair and never leave the baby unattended in his/her high chair as the baby could be hurt or even worse.