There are different modeling techniques that you can use to visuelise the relation between entites in the ERP. The best approach (and one I use) is to apply UML modeling technique. Good statring point to learn UML is with the tutorials on http://www.uml.org and http://www.tutorialspoint.com/uml/index.htm
UML defines multiple diagrams, I suggest you start with the Class diagram. The Class diargram discribes the relationship between the classes in the system, as well as the methods and attributes of the classes.
To draw the diagrams on your computer you will need a UML modeling tool. I use Sparx EA since it allows me to draw UML, BPMN and business processes flow charts. Though it's not expensive as a tool, might be "too much" for a start. I suggest you use some free UML modeling tools.