The first room I dig out is just storage space to get everything inside, this is where I will put some beds and assign a dormitory, as well as a couple tables and chairs for a dining room. This is what the dorfs will live in for the first couple seasons while everything else gets dug out. If it is a particularly vicious area, I will door/hatch the entrance. Eventually, my farms will go in this room, with a walled and covered area above on the surface for surface farming. The actual entrance will be elsewhere, so the only way to get to that room when the fort is set up is from below.
Having that room there grants me some time to think about the fort and entrance. I'm a fan of the walled courtyard behind a dry moat. The barracks will be first room inside the entrance (so everything has to go through it before reaching the rest of the fort) and likely be constructed above ground next to the courtyard. From the barracks to the entrance I will have pastured wardogs by the dozen. The courtyard itself likely won't be directly attached to the barracks, but instead be accessible from below like the farms, here I will pave the area with metals and set up a statue garden meeting area to stave off the cave adaptation. Since the courtyard is open air, it is important that the whole thing be on high ground, as I have had goblins camp on top of a nearby hill and shoot over the walls.
The top of the walls will be fortified and made accessible from the barracks. I usually make very vertical forts, so I have room to spread out and make a ring of fortified towers, again accessible from below, out and around the main walls. These help when an enemy siege will camp outside of bolt range from the main wall.
Often I dig out a soil layer to connect these towers, which can become a tree farm later on.
Once I have a good spot for the entrance picked out, I begin digging down from my temporary room over to that location, and then down further to where the workshops, bedrooms, dining hall, etc will be. Only after that is set up do I dig up to where the entrance will be. I may have a drawbridge closing the other temporary entrance until the real entrance is built. This should be completed in about a year, so barring a necro siege, I don't have much to worry about (and if a swarm of zombies do show up then....well I didn't stand a chance anyway)