There are many ways to subpoena. You should go to a LA self-help center to ask the attorneys there to prepare the document for you. Simply speaking, if you want to subpoena the witness' appearance (duces tecum), then yes you would have to schedule a court date first. Otherwise, you can subpoena the witness by deposition.
Please remember that the tenant is a third party witness, not a party in the fight. The tenant can file a motion to quash if the subpoena is invading his privacy or causing hardship. The court may modify the subpoena or quash it.
Did you have an evidence hearing on the issue? You ex' claim that he cannot work is not a valid reason to lower child support. The court may impute his income or order him to find a job. Child custody and child support are not something you can DIY.
