If you have the ABC software installed on your computer, then the process of creating BOMs is described in detail in the Help > Stating Up > Page 26 additionally the Help > Intermediate > Pages 26 & 34.
The main procedure to start creating BOMs is as follows:
Add assemblies and components into the Item Master first, then select [make] for the assemblies in the Main Supplied field in Item Master. Also add components to assemblies in Inventory / Bill of Materials Screen.