First of all, you have to identify the compound that have anti-diabetic properties (it will be your substrate) and then select the target protein on which the effect of this compound you want to study. You can do molecular docking by using AutoDock vina and other docking tools.
First of all find out genes and proteins involved in diabetes. Then download 3d structures pf those proteins which are involve in diabetes. Dock your plants compounds with those proteins in any docking saoftware you can also use online docking servers. Analuze docking results in discovery studio