First establish you are disabled. Only persons who are legally disabled qualify for a service dog.
Next, contact an organization that trains service dogs. Service Dog Central has an article with links to several lists of service dog trainers around the world, or simply contact Assistance Dogs International for the name of a member organization nearest you.
Though some in the U.S. choose to train their own service dogs or to have a dog trained privately, few have the skills to train such an advanced dog. Therefore most service dogs are from programs that specialize in training service dogs. In most countries other than the U.S. service animals are required to come from ADI accredited programs.