Actually, there are some software to create database model, for example Power Designer, ERWin, Toad data modeler, Microsoft Visio and etc . But I think after compared, power designer is the best. Why it is the best,it have good visualization and easy to modify about entity and relationship between entity. This tools have the good about easy to generate figure and sql script. You just choose kind of database, and it generated.
Sybase as developer this software have shared trial edition as long 15 days. you can download in http://download.sybase.com/eval/PowerDesigner/powerdesigner125_eval.exe. after that install into your PC, very easy to install because you just click next and next, very standard of windows software installation.
On this software you will know about :
a. Entity, entity is an object on system have some attribute or specification and it have relationship and activity doing in the system. Example : on schools system, teacher, student is an entity. symbol of attribute on Power designer is
b. Attribute, attribute is some specification and detailed spec of entity. for example Teacher have attributes : Name, teacher id, sex and etc.
c. Relationship, connectivity of between 2 entities