Don't Panic!
There is no such thing as a green thumb, there is only experience.
Forget this myth that plant lovers are born, instantly in tune with Mother Nature and that wherever they tread life springs up in their wake. That person does not exist.
This Shanghai Gardener has had a lot of experience killing plants (houseplants, outdoor plants and edible plants, not to mention expensive plants and those that are supposedly easy-to-grow), until finally arriving at a point where she can finally say she knows how to keep them alive. She grows things and they not only survive, they often thrive. You can do this too.
The most important thing to understand about plants is each one will have different needs. Once we understand what they want and actually give it to them, then they will reward us with plenty of healthy, strong growth.
So what do plants need?
The specifics are different for each plant, but they can be broken down as follows:
Soil
Ensure that the soil you use is fertile and not depleted of essential nutrients through many years of use. Add some
compost if you can, (this gardener adds hers annually each spring), and if you are growing hungry feeders such as citrus trees or tomatoes in pots, then consider adding a more regular fertilizer too. Good soil should also contain some air that allows the plant to “breathe”. Air in the soil can be depleted, especially by things such as over watering, so take care not to do that also. See below.
Water
Just like us plants need water to survive. How much or how little depends on the type of plant and the length of time it takes for the soil to dry out. One major reason why our plants die is from incorrect watering: either we love them to death by watering daily when they don’t need it, or we forget about them until they die of thirst. If you’re a busy person, maybe think about growing the types of plants that thrive with neglect. Be realistic about how much time and effort you really want to spend taking care of your plants.
Light
A general guide is that plants that grow fruit, many different types of vegetables, or plants that produce flowers will need plenty of direct sunlight (more than 6 hours). One of the reasons for failure at the beginning is not providing your plant with the light levels they require. Some indoor houseplants originate from the forest/jungle floors or shaded conditions. Putting them on a south facing window sill will at the very least burn them, or at worst kill them. Researching the light requirements for your new plants will help them to thrive.
Temperature
A plant that has been planted out of its natural season will grow weakly or not at all, and will be prone to all kinds of pests. A lot of our indoor plants originate in tropical jungles and need constant warmth all year long, while some vegetables such as broad beans and spinach, love a good chill to kick start their spring growth. Learn what temperatures your plants enjoy and grow accordingly.