Using a spray-on fertilizer is also helpful to many plants, but especially vegetables. You should only use it on vegetables when they are flowering and producing fruit. This will help them to produce more blossoms, which are necessary for it to bear fruit. Certain plants require a special plant food. As an example, azaleas need their own food because they need the soil to be acidic, but you need to watch it the first growing season because too much will burn the roots.
Sunlight is the easiest ingredient for a healthy plant.
Most plants require lots of sun, so they are very easy to satisfy, but there is also a group of shade loving plants as well. The easiest way to determine this is to read the labels on the plants when you buy them. The only hard part is that certain climates are too hot or too cold for certain plants. Usually, the plants you see at your local nursery are a safe bet for your climate, but if you are not sure, just ask. Plants like tomatoes require a lot of sun, whereas impatiens are a shade-loving flower.