Fortunately, most diaper rash is due to simple irritation of the skin from having the skin wet all the time. This is usually simple to clear up. It is important to change the diapers frequently, as having the baby stay in a wet diaper for a prolonged period of time will worsen the irritation. When changing the diaper, clean the skin thoroughly with a damp wipe, making sure to get all the creases. It is best to clean (and dry) by dabbing rather than rubbing, as rubbing tends to irritate the skin more.
After cleaning and drying the skin under the diapers, apply a thick occlusive ointment, such as vitamin A/D ointment, vaseline ointment, or zinc oxide ointment. Make sure to apply the ointment every time you change the diaper to maximize its effectiveness.
If this does not work, then you could talk to your pediatrician
, because at this point there may be sufficient inflammation that a steroid cream is needed. If there are very raw, red areas or multiple 'satellite' areas of redness moving outwards, these could be signs of a fungal infection, and an antifungal cream may also be needed. Your pediatrician can help you with these decisions.