If you want to be a SAP consultant, customizing will be a knowledge you must have. To have a competencies in customizing you must understand SAP business process.
Customizing covered following function:
1. Define Organization Structure
Business Area (Finance Accounting)
Maintain Storage Location (Material Management)
2. Maintain Master Data
-Define Customer Group (Sales and Distribution)
-Define required entry in each module
4. Output Layout
-Define output layout
How to customize SAP
Go to TCode SPRO then click "SAP Reference IMG".
You will see tree of customizing Menu.
For example we want to maintain Sales Group, double click this menu from tree, you will see table maintenance view to maintain master data.
To have a competencies in customizing you must understand SAP business process.