We are in the process of having a new home built, and I could probably just say I am learning A LOT - but I think instead I will list some of the things for others who might be building or contemplating the process.
Number one thing is to choose contractors you will be able to work with well! I am really fortunate in having contractors that I have worked with for years, both as they did renovations for clients and also remodeled our own current bathrooms. So I know their working style, their quality of work, and their communication preferences (and they know mine....and still agreed to build for us!). If you do not have this luxury, then make sure you get lots of recommendations from people you trust, check out their credentials carefully, and have a conference where you can gauge if your style & theirs will dovetail nicely - or drive you crazy during the process.
I think the next important thing is to spend some time really thinking about how you live, what changes you want to make in a new house, and what things are SUPER important to you vs things you don't have strong opinions about one way or another. My husband was initially unsure that the square footage I was suggesting was big enough until we really talked about which spaces in our current house we use.....or not.
We agreed we wanted to move to a smaller home with plenty of space for all the necessary things, but without wasted space. We knew that a one level house with an open main living area was important. We knew that lots of light, energy efficiency, a home with "green" products were all things that we valued and wanted.
I have literally made a list of furniture that I know we want to keep, along with dimensions. There is also an ever changing list of other possessions that we use or treasure and how best we want them to be displayed or stored. All of this information is really important to think through before you attempt to design a floor plan, or think about number of closets, types of cabinets etc etc.
Be prepared to be excited, frustrated, happy, terrified, and about every other emotion under the sun - building a new house is creating a new life style and it is truly a big deal! Thankfully, the positive emotions have WAY outweighed the middle of the night panic attacks or frustrations over WHEN will the weather cooperate.
We are now at the stage where the house is ready for dry wall, and I can walk through rooms and clearly imagine how they will look finished. It is exciting to see things come together as planned (and sometimes better than anticipated). Bonus points that since I pretty much drive by daily just to see what is new, I have met almost all my soon to be new neighbors!
So....lots of interior finish work still must happen, but I can see that completion is not all that far off now - and so now my main panic moments are about how much sorting I want to complete before the move!