A biosphere is a closed system meaning nothing but light comes in or out. You can build your own biosphere by filling a bottle or jar, preferably glass or very clear plastic with items from an ecosystem like the forest or a water body. Make sure that you include, producers organisms that produce their own food (plants).
Also include some of soil there are many small organisms that act as consumers and decomposers in soil. Consumers eat plants and animals, decomposers eat dead things and turn them back to soil.
If it is terrestrial or from the land, be sure to water it before you close it up. You should make sure there is some drainage or irrigation process, so putting some gravel at the bottom of the jar will help with this.
For the aquatic or water biosphere, be sure to give it time to settle it will look drastically different after just one day.
Some biospheres are a good balance and can last for many, many years. Some may only last for a short time and they start to get off balance and grow molds and too many fungus. If this happens it is best to clean it out and try again.