How are anti-depressants prescribed?
It'll be different dependent on the physician/psychologist on that case. But here is my best shot at it.
The main use for antidepressants is for treating clinical depression (usually in adolescents and adults)
In most cases antidepressants are given as a first for of treatment (SSRI's typically being the first tried) Typically in tandem with a talking therapy such as Cognitive Behavioral Therapy (CBT)
Yet in children and young people with depression, are in most cases offered a course of psychotherapy that typically lasts for at least three months time. In some cases an SSRI may be offered in tandem with psychotherapy for moderate to severe depression in ages 12-18.
They can also be used to help treat other mental health conditions, including the following
Some forms of phobias, such as Agoraphobia and social phobia
As with depression, SSRI's are usually the first line of treatment with this conditions. If SSRI's prove ineffective, other types of antidepressants may be used such as SNRI's/NDRI's/Atypical Antidepressants/Tricyclic antidepressants (TCA's)/MAOI's.
And in some cases, TCA's (Tricyclic Antidepressants) can be used for neuropathic pain, typically when they are unresponsive to NSAID's, The main one used in this class would be Amityiptyline.
Hope this helps a tad.