First, pause. Then look over surface and find best place for entrance (I like it close to the wagon but sometimes there is a better spot). Designate mining of entrance tunnel to fort where depot will be located and defensive path to rest of fort. Then dig two big rooms in level below depot or on depot level. These in the long run will be storage rooms for trading with the depot (transfer all junk to these rooms so easy to find when trading as they are closest). Then designate a tunnel going to the center of the earth to find the magma layers and water from the caves.
Then I designate a pasture, a huge wood stockpile, a stone stockpile, and designate all the trees near the entrance to be cut down. Unpause.
Once stone is found and trees cut, I build a mechanics shop, mason, and carpenter on the surface. Carpenter makes 6 wheelbarrows and some beds. Mason makes a few tables, chairs, and blocks of the color I want for the depot and floors of the entrance. Mechanic makes 3 mechanisms. Then I build a wooden wall around the entrance with a gate and put the pasture for animals on the outside of the gate (I don't like werebeast visits in the first few years and don't start an army for three or four years, so I give them some bait and a way to keep them out). I also move all the stuff except for the vast amount of coal I bring on each embark from the wagon into the two storage rooms. I am very careful in doing this so to keep everything separated (all types of food in each of their own pile, cloth in cloth pile, etc.; its a pain to separate bins or barrels later on). I then build a dorm in a corner of one of the rooms and a little meeting hall in the other. I also immediately wall off any breaches to the caves. Early troll means no more fort. I set up a stonecrafter and rock stockpile in one of the store rooms and build the depot (I don't pack enough food to make it without the first caravan; the first migrant I get becomes the stonecrafter and that's all they do until the rock situation is taken care of, silly dwarf crafts = food).
By early summer, I usually have a little wooden castle with a gate for protection, a depot, simple storage, manufacturing, sleeping areas and a meeting/eating area. Best of all, I typically have a good idea if and where is water (if no water, no luck!), what colors of rocks there are (important for decoration and castle building), and what types of ores there are and if there is coal. At that point I pause and layout all the digging I will do and have the miner(s) start with the farming rooms, then production, then storage, then meeting rooms, then bedrooms, and after that, doesn't really matter the order. After all that digging, smoothing, decorating, etc. is done (I do run the farm as soon as possible though, as boos are necessary), I empty out the storage rooms, tear down the wooden castle and above ground shops, finally take apart the wagon, and build a cool castle around the entrance.
Works pretty well unless some troll gets in when bypassing the caves...