Also, I have gotten three migrations so up to about 17 dwarfs and then a slew of children. It's about this time that I get overwhelmed (lol) and don't know which direction to go.
Just keep things simple. You should have small areas dedicated to the essentials of Dwarven life in the first year, but with room to expand them as more dwarves arrive.
1) Food and booze. Have some
small underground farms in your soil layers (I use 2x2 plots, and I start with 4 of them planted, but with another 4 plots sitting fallow). Put the seed stockpile (a food stockpile that disables everything but seeds) with NO barrels allowed right by the farms. Make a plant stockpile close by, for the harvested crops to go to. Use that as the center for your plant-based industries (brewable plants go to the still, pig tails go to the farmer's workshop -> loom -> clothier's shop, and quarry bushes go to the farmer's workshop -> kitchen).
Note: quarry bushes have two bugs in the current release (0.40.08). There are workaround instructions in
Bug #7032. You could also just skip quarry bushes, but that means you'll need additional farm plot space for food.
2) Security. Have a way to seal yourself in, for when the zombies (or goblins, or whatever) arrive. A raising bridge that blocks the sole entry is the most common way. You can devise more elaborate/devious designs later, when you have the time. Have a few military dwarves training, and add more as you get migrants that you don't need for civilian duties.
3) Water. For medical use only. Ideally you can locate an infinite water source (brook, river, aquifer, or cavern lake that touches the map edge), and divert some of that to a private underground cistern, with wells over it. Having the cistern underground prevents it from freezing. This is a one-time setup per fortress, typically. Once you have this, you shouldn't ever need to expand it.
4) Bedrooms, dining rooms. This is about happiness management, which is important in the long term. In the initial months, you can get by with a communal dormitory, and let dwarves eat food straight out of the stockpile. Later, you may want to build individual bedrooms. Building a nice dining room gives a happy thought every time Dwarves eat there. It really doesn't take much to impress them, either. Half a dozen good quality stone tables and chairs in a single 8x8 (or so) room will do.
5) Clothing. This becomes critical after 2 years, when the first batch of clothing your Dwarves arrived in starts to wear out. Expand your pig tail farm plots to meet the demand, or add some above-ground textile plants (rope reed, cotton, hemp) if you can get them, and if your biome can grow them. Remember that pig tails only grow for half the year (summer and fall). You can also use leather for clothing, but locally sourced leather is
really difficult to get in bulk. If you can get the Dwarven caravan to arrive regularly (no sieges preventing them), you can get lots of leather that way, but pig tail plots are the most reliable source of clothing.
Above all, don't panic. You get 30 more migrants, no biggie. Just carve out 30 more rooms, chop a few trees, build 30 more beds, and voila. Maybe add another 2x2 farm plot to support their drinking habits and slaughter a boar. You should always have enough extra food and booze to hold you until the new crops grow. The new migrants' clothing should be OK for 2 years, so you have time there. They should arrive "content", so the lack of a bedroom isn't going to send them into a deep depression right away.
As noted, cook is turned off for all seeds so as not to consume them. So I don't know where my seeds are going. I brought over 100 with me from the start. Plots are also rotated seasonally.
Which plants? This is important, because quarry bushes are broken (but fixable), and many of the above-ground plants simply don't work at all (also theoretically fixable, but the modding required would be more extensive -- basically implementing them yourself as brand new plants).
Remember, you don't get the seeds back until the mature plants are processed or eaten. If all your plump helmet spawn are planted, grow into plump helmets, and sit in a stockpile somewhere, then you won't have any seeds left. When a dwarf brews one, or eats one, then you get the seeds.