How and when to collect seed from my garden plants? What about savin
First some basics. This may seem obvious, but you must leave flower heads on plants that you want to set seed. Deadheading to prolong bloom is not an option unless the plant produces enough flower stalks that you can deadhead some and leave others to fall off naturally while seeds form. If you grow vegetables and fruits to eat, allowing plants to go to seed may spoil the intended product (for example, lettuce leaves taste bitter once plants flower) or reduce the quantity of edibles, or the seeds themselves may be what you plan on eating (peas, for instance). A further consideration is that many popular garden plants, grown for flowers or for food, are hybrids and either do not produce seed or their seeds are not viable (viable means able to germinate and grow). In any case, before you can collect seed from any plant it must go through the whole cycle of flowering, being pollinated, developing seed, and ripening.
Some plants develop seeds within pods of varying descriptions, others hide their seeds inside fruits or berries, and still others harbor their seeds directly inside the dying flower heads. In all cases, any seeds produced will develop in close proximity to the spot where the flowers originally appeared. If you can not locate seeds on a particular plant, try posting a question to the Growing From Seed forum here at GardenWeb for assistance.
It's best to let seeds ripen on the plants until they are *almost* ready to disperse on their own. It's a common mistake to separate seeds from the plant before they've finished developing; any collected too early will not be viable. Once seeds have finished enlarging, they will generally change color (from whitish or green to tan, brown, or black) and begin to dry out. Pods will start splitting open, berries or fruits will shrink and wrinkle, or flower heads will begin to fall apart, dropping the seeds within. On many plants the seeds don't ripen all at once (check daily), and it can be useful to tie a small paper bag or the toe cut from panty hose over the seed heads to prevent dispersal before you can harvest. If this method isn't practical, collect the seed heads or pods before they are completely dried out (i.e. papery dry), or the seed may either have dispersed or will fall out when you shake the plant stems while breaking off the pods or heads. *Cutting* the stems will help to prevent this sort of seed loss. As long as you wait until the flower seed heads have turned dark and fairly dry, or until the bracts ("leafy" shape that surrounds the petals of a flower where it attaches to its stem) or the pods themselves have quit enlarging and begun to dry out, you can cut and take these indoors to finish ripening in a cool, shaded place inside a closed paper bag (this will prevent you from losing seeds which disperse *explosively*, such as Impatiens and Phlox).
How do I know if my plants are hybrids that may not produce seed true to the parent plant?
Always save any tags or labels on plants you buy and ask questions about plants you obtain from other gardeners. Unfortunately, many commercial growers don't tag their plants very informatively (even the conscientious ones make mistakes), and many gardeners only know their plants by common names, which can vary widely throughout different parts of the country and world. Your best bet is to learn all you can about the plants you have or want and post questions about specific ones on the Growing From Seed forum to get additional advice.