You can only bet on horses at 1 of 2 locations in San Andreas onme in Downtown San Andreas and one in Montgomery. The Montgomery location will be revealed during the regular story missions with crazy Catalina. Once you find and enter one of the Inside Track betting locations you simply walk up to an ITB machine to place a bet on a horse. Your horse must win the race to unlock the trophy and the winner is random. Still it seems that the odds do have an impact so best to bet on a high odd to win horse (3/1 or 5/1 etc.)