All good advice above.
The wiki suggests a single 5*5 plot, which works fine until you have a significant number of dorfs. Generally, by your second immigrant wave, you should open up another one. At first, just go for plump helmets because they're a good starter food.
I go with all 6 starter (UG) seed types, not only for diversity in alcohols, but also because you get a free bag with each type of seed purchase. And some of the more complex ones allow your dwarves to Mill later on, which is good for their attributes. (Even Dimple Cups, which are non-edible, just to have them when I want them later.)
I go with a 2x4 or 3x3 plot for Plump Helmets (and Quarry Bushes, tho' those are another "advanced" crop that needs a Farmer's Workshop), and a 2x2 (or 1x4) or 2x3 each of Pit Tails (Ale), Cave Wheat (Beer), and Sweet Pods (Rum). That, with some odd berries or whatever from early (unskilled) Plant Gathering for AG (Above Ground) crops, maybe 2x2 plots (a bit larger if more valuable, smaller if a garbage crop) and you're set for up to 50* dwarves or more, no prob.
If I can put them all in a line, there are spaces to store the seeds in front of the smaller plots - one tile/seed type, right in front of the crop.
This total is about equivalent to the 5x5 plot - call it ~17 for the PH + QB, + ~5 each for the other 3, = ~32 tiles or so, plus the AG crops.
Don't be tempted to make the farm plots much bigger - it's wasted effort to plant and certainly to harvest, and your stores will overflow with uneaten vegies or oceans of booze. Which may seem like a good thing now, but it's just a different headache. (But if you're going to go with a big fortress eventually, plan ahead and leave room for expansion. You don't want to have to flood your crops later all over again.)
(* Note - this assumes you have a Proficient (+5 levels) Grower. This helps a LOT, but is not a dealbreaker - just up the size of the farm plots, maybe +50%, no more than double.)
I use the above numbers with a Proficient Grower on embark, and often have ample to cook a bit of PH Wine just to get rid of it. If I'm lucky on AG crops, various berries that make even more booze, I sometimes allow the largest type to be cooked. Elk roast with minced plump helmet and quarrybush leaves in prickleberry wine sauce - even sounds good. But I micromanage, and don't "automate" my foodstocks, admittedly.