I use certain groups in Therapist. My current setup:
Farmy: farms and cuts trees (has an axe)
Worker: Digs, builds things of stone (has a pick), also building objects because in the beginning only the mechies won't do.
Crafty: Biggest group, contains all create-things you use only rarely, and mostly they build wood stuff of detail walls/floors
Mechy: Mechanics, pump operation and building buildings and traps
Foody: Butchers, cooks and brews
Fishy: Fishes. I never have more than 3 since I always overflow with food
Hauler: No jobs, only haul stuff. Only group that buries.
Smithy: Has all 5 metalworking categories on, so makes stuff of metal, smelts stuff and deals with glass
Then I have some specialist groups for nobles (no tasks, but the bookkeeper, broker, doctor and mayor are in there) and soldiers or marksdwarves.
I try to keep the task groups around 12 dwarves each, and the rest are haulers. I try to assign them based on their skill, but otherwise they'll level themselves up. Only with Workers I am a bit more cautious, so they don't waste too much ore. I never have any problems. And migrants are initially put in hauler until I decide to make another group of recruits or some dwarves die.
As for food, a small 4x4 farm can probably feed over a hundred dwarves if you combine it with some fishing and an occasional butchered cow. Though it helps to grow some variety, especially aboveground crop is good for booze (whip vines, longland grass and strawberries, easily acquired by gathering or from the elves). I have about 50 squares in use for a variety of crop, and my fortress is exploding with food. I sell lavish meals to buy stuff I can melt, so I have access to more metals.
For finding ore, you could use dfhack's reveal to see how layers are composed, and use that knowledge to learn about the relation between layers and ores. I usually find a massive amount of ores, coal and other stuff, although it's mostly between layers 5 and 15, the rest is gold and other odd ores. Having flux is good too, so you can make steel.