Babies up to 1 year of age should always be put to sleep in their crib on their backs.
The reason the baby should sleep on his/her back to is because of the risk of SIDS Sudden infant death syndrome.
A baby 1 year or under that sleeps on his/her side is at higher risk for sudden infant death syndrome so that's why it's really important for your baby to sleep on it's back up to 1 year of age.