Gardening?

I am the absolute worst at the gardening skill. I'd recommend that you Sim goes to the library and reads the skill book on gardening first of all. Once you're at level 1 gardening skill, you get given a bunch of seeds in your Sims inventory which you can plant by clicking on them and selecting plant/plant many. You can then click where you want your Sim to plant the plants. I recommend you read the library book until you can't learn anything new from it so that you're able to weed and fertilise your plants as this will increase the quality of the produce.

You can find seeds all over the town. They're small, beige/brown in colour and shaped kind of like tear drops. You can find loads on the floor. A great place to find them is the park where several spawn. Your Sim has to be a certain gardening level before they can plant seeds such as uncommon seeds so you're better off starting with common seeds like tomato, grape, lettuce and apple first of all.

When your plant starts to grow you can select tend garden and your Sim will water the plants and weed them if their skill is high enough. Make sure to do this every day and produce will grow.

On top of what Dynamite said, you naturally build up your gardening skill as your Sims tends to their garden and they can also take a class at the Science lab in Gardening. Personally I think it's more beneficial to increase your skill before you start gardening because you'll be able to tend to your plants better and you'll have better produce which will be ideal for cooking and improving your meals.