Design is NEVER easy, so asking for an easy site is a random thing. "Easy" to me will be "hard" to others and vice versa, because each of us sees things in a different light. Each of us is influenced by our differing backgrounds.
With regard to Entity Relationship Diagrams, the trick is to define the entities first, then define how they relate, then draw the picture. (Which you could have figured by just looking at what ERD really means.) But the REAL starting point is always, always, ALWAYS to know your problem before you try to identify its components, relationships, and how you need to "see" it. There is no substitute for the human thought process when it comes to problem analysis.
So... an EASY ERD site or text? Search the web, sample some sites, and tell us what you found. You won't be back soon unless you have a low threshold of impatience. But that is not meant to dismiss you, it is just to emphasize that you ask for something that might not really exist.
