Take a look at the Newbie Central forums sticky threads, and the stickies in this forum.
Overwatering is largely due to a lack of oxygen in the rhizosphere, which plant roots breath. As a result the leaves will go almost droopy but curl under. Water contains only minute amounts of dissolved oxygen, which root will quickly deplete. There are a couple reasons why it can occur: either because you're watering too frequently and not allowing the mix to drain, the pots themselves are sitting in standing water, or the mix contains too many fine particles and therefore becomes water lodged and drains improperly (or not at all).
It is much more difficult to over-water if you have an airy medium with good drainage. Soil and soil-less potting mixes will often utilize various amendments including perlite, vermiculite, coco coir, sphagnum peat moss as these materials are both porous and absorbent; they hold air and water. Even with a good potting soil or mix it is advisable to add a little more perlite or coir for drainage. If you have a good draining mix then just don't let the pots sit in stagnant water for any prolonged period of time.
When you do water you want to water thoroughly, so that some solution comes out of the drainage holes. You water when the top inch or so of your soil/mix is dried out, you don't want to let it dry out entirely. A good way to help determine when to water is to check the weight of the container. After you water they will be quite heavy, and lighten up considerably once it is time to water. Smaller containers need to be watered more frequently, and as a plant and it's root system grows it will also require more frequent watering until it is transplanted into a larger container and the roots begin to stretch out once more. The leaves of an under-watered plant will begin to wilt until going plain limp.
As for fertilizing, it is a bit more complicated as deficiencies will present different symptoms in specific parts of the plant depending on the nutrient that is lacking. Multiple deficiencies may present when the pH is not in the appropriate range (for soil between 6-7), even though the nutrients themselves may actually be present within the medium- they become locked out and thus unavailable to plant roots. Different nutrients become more or less available depending on the pH, and other things can help plants absorb nutrients outside of the proper pH such as symbiotic fungi and chelates both organic and synthetic. Familiarize yourself with the nutrients plants require: nitrogen, phosphorous, potassium, calcium, magnesium, sulfur and trace minerals or micronutrients (iron, boron, copper, manganese, zinc, molybdenum, silicon, et al). Deficiencies present with symptoms in different parts of the plant depending on if the nutrient is mobile, immobile or semi-mobile. Nitrogen for instance is mobile; when deficient the lower, older growth will become chlorotic (turn yellow) as the nitrogen moves to the newer growth where it is needed.
https://www.rollitup.org/general-marijuana-growing/1469-marijuana-plant-problems-cures.html
