Basically i have two database tables

Patient
Patient Number | Patient Last Name | Patient First Name | Patient Address


Patient Charges
Item Description | Item Code | Patient Number | Amount

How do I make a diagram showing the relationship between the entities Patient and Patient Charges?

Try Powerpoint. I find it very useful more making simple diagrams.

Dia may be an option for you - the price is right, it works on Linux & Windows, etc. It does general purpose diagramming, flow-charting, network layout, and ERD diagrams among others.

Dia - GNOME Live!