Roots are everything

At the beginning of a plant’s life, or its first step towards becoming a self-sufficient plant after cloning, it must have roots. A seed first produces a root, and a clone actually reassigns cells in its stem to change into root cells. Roots, in order to grow healthy, productive plants, must have oxygen, nutrient, and freedom from disease, insect, or bacterial damage. Those are the things that growers can control.
When plants wilt, many growers assume it is lack of water. They water, and the plants look worse. If they check their pots, the growers discover heavy, soggy soil. And the plants die. Lack of oxygen in the root zone is the cause. Light, frequent watering is the answer. As the roots absorb the water from the soil, air is drawn down into the soil. (Air is better supplied with a light airy mix; supplement denser soils with perlite or grostones.) Poking holes through the soil with a pencil can help open air passages. Use a light hand when transplanting.
Nutrients are valuable to your plants, of course, but some are more beneficial for the roots. A good, well-balanced nutrient program will handle this. Enzymes are important to add as a supplement that greatly benefits the roots. Senyzym, Hydrozym, and other such products deliver tiny little cleaning machines to the soil. As root bits die off, the enzymes chew it up and return it as nutrients, instead of letting the root bits rot. They also are great at destroying harmful bacteria. Clean roots are healthy roots.
Disease is a matter of cleanliness and quality of your products. Diseased plants are non-productive and will kill your whole garden. Usually culling(throwing the plant away) is the best option for a beginner.
Some people say that insect infestation, especially spider mites and fungus gnats, are impossible to prevent. Cleanliness again. But if you do get them, act right away. First of all, have a 10X or stronger lighted magnifier so you can inspect your plants. Identify the culprit, and respond accordingly.
Lastly, providing beneficial bacteria and fungus to your roots with really enhance the roots’ growth and effectiveness. Beneficial bacteria actually grow around the roots, protecting them from attacks by destructive bacteria. The fungus will grow microscopic chains of cells, some miles long, which grow from the roots and transmit nutrients directly to the roots. The fungus can’t survive on these nutrients; they need carbohydrates to grow. The plant provides these carbs through photosynthesis. This symbiotic relationship is good for the plants and the fungus. Great White, Plant Success, and Orca are a few sources.