Discuss this first with your medical caregivers. Do they agree that you are legally disabled (under the ADA) and you need a service dog? You will probably need their support to get the medical documentation a training program would require of you.
Do you have the facilities and financial resources to care for a service dog? Do a budget. Are you able to care for the dog yourself? These are important considerations.
Make a list of the things you cannot do for yourself and write up a paragraph or two describing your lifestyle (are you active or sedentary, for example). Do this before approaching an agency so you'll be prepared to answer their questions and ask some of your own.
Start thinking about what it is that you want this service dog to do to mitigate your disability. In order to be a service dog, the animal must be "individually trained" to "perform one or more tasks which mitigate the disability."
The following do NOT count as trained tasks:
-companionship (even for agoraphobia or anxiety)
The dog has to actively do something, that you cannot do for yourself,
that also lessens the effects of your disability on your ability to function in the area of major life activities.