General painting advice

Hi,

Fairly new to decorating (as my numerous posts have probably made evident) and want to do as good a job as a novice realistically can, both for finish quality and longevity.

We're in a new build, the walls are just white. No idea what paint was used but I'm guessing something that allowed the building to dry. Is it likely the walls will have been primed prior to that. paint going on

What steps do I actually need to carry out? If it's unlikely to have been primed do I need to

Prime>sand>undercoat>sand>top coats?

Thanks,

Luke