The first line of treatment should be an emollient. There are zillions out there so it's a question of trial and error to see what works. Doublebase, Epaderm, Cetraben, Diprobase, Aveeno are common ones. Avoid aqueous cream and E45 as they can make it worse.
Also ditch all bath products like J&J etc, instead use Oilatum or Aveeno (again trial and error to see what's best).
Use 100% cotton clothing, stop using fabric conditioner, and keep LO's nails short to minimise scratching.
If it gets worse then you will need to go to your GP for a mild steroid cream. If you get weepy patches then it's infected and needs a cream with antibiotics in.
Some people find eggs or dairy make it worse so if you are BF you might want to try an exclusion diet, this needs to be done properly (ie absolutely NO products with these in, like whey, lecithin etc) and needs to be for at least two weeks to see if there's an improvement.