How is serotonin released?

Complex. It is a neurotransmitter, mostly in the brain and the gi system. It is released when that neuron electrically achieves an action potential, based on its interactions with other neurons. Those interactions are complex. What is more complex is the many different kinds of receptors, which do many different things.
Your own cells. There are some precursors like L Tryptophane but your own neurons will synthesize it in very selective areas where this neurotransmission takes place. FkFl.

Related Questions

If SSRI's block reuptake of serotonin, when/how often is serotonin released? Since changes take a month, is it infrequent? Or you just need sooo much?

It's more complicate. When a neuron fires, after it does so it will reabsorb or "reuptake" some of the serotonin it released when it fired. To inhibit this process, the neuron needs to change so it has fewer openings available at that end. The drug causes some of the openings to close, a change which takes a while. It's called "down-regulating." Read more...