Cultivation - My plant is not ok, why and what should I do? A few advices.

[genaro]

...I will assume that
-you've placed small rock or clay balls with a piece of tissue over them in the bottom of the pot so to provide good drainage.
-you have choosed a substrate that is appropriate to the kind of plant you're growing

*the plant doesn't like full direct full sun and has too much of it, or (most common case) the plant doesn't recieve enough light:
place it in a very bright place (natural light or under horticultural lamp) but not in directly under full sun (for instance in a place of your garden which is very bright all day long but only recieve direct sun for a few hours per day, not all day long)

*the plant hasn't enough water, or worse (most common case) it has too much of it (which is especially bad during winter if the plant is outdoor in the frost):
give water to the plant in abundance, but only when the compost is dry to the touch. If parts of the plant have started to rot due to the excess of water (especially roots), cut the rotten parts. If the roots have started to rot, maybe it's also a good idea to repot the plant in new compost to avoid infection

*It's winter, weather is getting very cold (frost, temperatures of O°C or under) and the plant isn't a hardy one (meaning it can't handle frost): take it back indoor for all duration of the cold season (place it in front of a very bright window, as much as possible a south facing one...or under horticultural lamp)
If parts of the plant have started to rot due to frost and water (especially roots), cut the rotten parts. If the roots have started to rot, maybe it's also a good idea to repot the plant in new compost to avoid infection.

*you're giving too much fertilizer to the plant during its period of rest (winter) or while it's still to young for that (young cutting/seedling) and it's burning the folliage: stop giving fertilizer for a while and see if things gets better.

*tropical plant that needs high hygrometry (high air humidity) that is grown in a dry place:
place it in a small greenhouse to retain more humidity.

*plant is pot bound (and therefore growth has stopped): it needs repotting (change the compost for new one and choose a bigger pot)

*the plant is affected by an illness:
The first point is to identify the kind of illness: parasitic insects? mycosis? bacterial infection?

Carefully examine stems and every leaf of your plant (both faces) so to detect the presence of small holes or miscroscopical inscects/eggs, or spider's web. Also search for abnormal staining, fading, wilting, mycosis, rot or so.
Also, take the plant out of its pot and both check its roots for the same kind of things and examine the compost searching for any microscopic insect invasion.

If leaves are affected by insects, don't cut these, wash the plant folliage using a water jet (or under the shower), allow to dry and then just apply appropriate treatment (both over and under the leaves, of course).
If leaves show signs of a mycosis, cut the ill ones and apply approppriate treatment on the folliage (if the plant is severly contaminated, better separate it from your other plants to avoid propagation by spores)
If an insect invasion is found in the compost, repot the plant in new compost.
If roots show signs of mycosis or rot, cut the ill roots, and repot in new compost and using a new pot (the old pot should be discarded or sterilized) and apply antifungal treatment.
If the problem of your plant turns out to be some kind of bacterial infection, you must immediately separate the plant from your other plants, and cut the ill parts (leaves, roots) AND change the compost and use a new pot (old pot should be discarded or thoroughly sterilized) and rapidly apply approppriate treatment.


About treatments: always start with some light, low toxicity treatment (rather than using some powerful high toxicity stuff). You can find many kinds of so called polyvalent/total/multi-use treatments sold as spray bottles, which are low toxicity and designed to fight many kind of illness at the same time (meaning that even if you couldn't identify the exact nature of the illness, there is still a great chance that the spray will solve the problem)
Also, note that copper sulfate and sulfur are sold as antifungal and disinfectant for plants, and these can also be considered as a not-too-agressive kind of treatment.
DO NOT HARVEST THE PLANT FOR AT LEAST ONE MONTH AFTER THE USE OF ANY KIND OF TREATMENT, and always rinse the folliage with a water jet many times before harvesting.
Also, during the few weeks of treatment, it's best to protect the treated folliage from rain so the product is not washed of.