WES2006AG is spot on, as is Milkman. The umpires for Baseball USA (Marucci), Premier Baseball, and a few others are assigned through CBCU. The umpires for public high school teams and a fair number of private schools throughout greater Houston are assigned through TASO.
I am a TASO high school umpire, also with CBCU, and do local Little League games. Reach out to Ken and Charlie. The TASO new umpires class is free, but to umpire high school baseball you will have to pay local chapter dues and state dues, attend some meetings, and umpire a few scrimmages to get started. CBCU will ask you to pay some annual dues, and both also have some scheduling fees per games that they skim off.
What part of town do you live in? There are also some niche umpire groups like Spring-Klein Sports Association (
SKSA), Fort Bend Baseball League, Lonestar Baseball Umpire Association (LBUA), and others that may be of interest to you.