If you have lasted this long, then you undoubtedly have already been doing things like hunting, fishing, gathering, and trading to get food. Keep doing that even after you start farming. A single staple crop might keep your dwarves alive, but having variety will keep them happy. In fact, one important thing to keep trading for is booze, even if you already have plenty. You will find that not only is more variety in booze just as important as variety in food, but all booze comes in barrels and you will find good use for them, and here is why:
As has been said, you will find that farms becomes more productive over time as your planters gain more experience through harvesting. This can actually be a big issue for you if you are not prepared for it. Much like the American midwest during the Great Depression, the excess of food production can actually wreck your overall economy, unless you take steps to accont for this. The number one thing that will happen is that your barrels will all go to storing food, which limits your ability to brew and store booze. That in turn causes all dwarves to slow down their duties and become unhappy, and if you are not paying attention you might realize this too late.
Some steps to mitigate such overproduction are for starters set any booze stockpiles to have a certain mininum number of barrels reserved for them, which will allow at least some booze to be brewed and stored at all times. Also, keep your barrel production going, with either wood or metal or both as you can manage. You do not need to go overboard, but do keep occasionally turning out more. Keep a stockpile reserved for only empty barrels to account for them, as their number will fluctuate up and down as barrels are filled and emptied.
Additionally, when your food reserves start getting high, use your job manager to start producing large orders of lavish meals. This will use up some of the food and consume some of the seeds/spores to slow the farming a bit, as well as compressing that food into smaller stacks and giving happy thoughts to dwarves who eat them. Plus they are high value, allowing you a lot of trade power with them. I also recommend starting to rotate your crop production, so you produce less total eatibles, and more things that require more stages to go through to be used.