Once you embark, take a few minutes with the game paused to scout around the map and figure out where you are going to build and get a general sense of what kind of fortress you want - aboveground, deep, shallow and sprawling, etc. My first order of business is ALWAYS identifying where I think the underground farms will go and building around that spot.
Then consider the resources you have available - trees, obsidian, magma and figure out what kind of economy you are going to start out with. You need to trade *something* in the beginning. Pick one thing and build around that. Don't worry about bedrooms and such in the beginning. You can always plop 4 beds down in the corner of a room and let them hotbunk for a season, and later remove them to use the room for what you intended.
Once you figure out your early economy and farms and some basic fortress design, build one component of that design. Maybe the farms and a trade hall where you'll have your depot and finished goods later. Dig only that out. 7 dwarves don't need much space. I might dig out a 9x9 farmable area, build a 9x1 farm plot, and stick my brewer, kitchen, and butcher shop in there for the beginning along with a small stockpile. Get your basic economy up and running in the smallest reasonable space - you'll minimize hauling jobs, which are what kill small population fortresses. Once everything is inside and food is planted, *then* plan out your expansion.
Give one miner the task so he'll become legendary as soon as possible. Once he's legendary, make him a hauler (or some other job) and skill up another guy. Don't give him 3000 tiles to dig out, or he'll sprawl all over the place and yield no usable space until he's done. Give him 200-300 tiles to dig, wait until they are done, then give him 200-300 more. That way you'll get space to expand on a regular basis. You can then take a room and just stuff in one of each workshop, just so that you have them when you need them or if a mood strikes. You can take them down later when things are laid out better.
My advise on a basic economy would be prepared food. If you get an immigrant rush, you'll need it desperately, and with easy meals your cook will skill up pretty fast and those things will have a decent value. It's also an economy that doesn't take much space, works even when under attack, and allows you to slow the pace of things down a bit so you don't always feel rushed. Plus, you need those buildings and skills anyway.
As for overall layout, I tend to organize things by z-level. One level will have all workshops, or all workshops of a certain type (all food/plant, all stone, etc.) Above and below 1-2 z levels will be storage for those shops. Dwarves can walk down 10 flights of stairs as quickly as they can cross a 10-wide room. Take advantage of that. I can fit 200 dwarves and a full economy (4-8 of every building) VERY comfortably in a 50x50 area. That includes 2x3 bedrooms, farms, storage, tombs, noble quarters, etc. It'll be 20-25 stories tall, but a dwarf can walk from one end to the other in no time.