newborns often favour turning their head to one side over the other which as they spend a lot of time on thir back can lead to a flat head. it will often resolve as they become more aware of their surrounds and look round more.
sometimes they can have a tightness in one of the muscles in their neck which can prevent/make it harder for them to turn their head in the opposite direction (torticollis) so it may be worth a trip to your gp to check it out as a referral to paediatric physio would be needed if this is the case.
to help flat head lots of tummy time when baby is awake and supervised is good, it can be with baby laying on you while you are laying on the bed/floor/sofa as it feels secure to them and as they gain control of their head will be lifting their head to look at you. as they gain more head control you can try them on their tummy on the floor and maybe use a rolled small towel/blanket under their chest/arms to lift them off the floor a little at first.
once they can hold their heads up well they can sit in a bumbo for short periods too