You have the documentation here 

http://world.episerver.com/documentation/

I would recommend starting by installing the alloy template site and then read editor guide, admin guide and then the developer documentation.

If you install a demo site you also get a database you can check out. See some example sql queries here (for older version though so might be a small change or two)

http://world.episerver.com/blogs/K-Khan-/Dates/2013/4/EpiServer-CMS-6-Useful-SQL-Queries/

Normally though, you never need to mess with the database directly. You use the API for that. Useful classes is the old Datafactory that can get and save pages and the newer ICONTENTREPOSITORY which does the same for newer versions of Episerver. With that you can create pages programmatically and that is what is used in the background when editors publish new content from the edit mode.

While it can be interesting to know how it works internally, I agree with Daniel that you should get to know the API:s first, mostly IContentRepository/IContentLoader, how to define a strongly typed type content, how to work with block etc etc - trust me, there are enough concepts to keep you occupied in weeks, if not months to fully understand them.

Know how data is stored is pretty low-level and only helpful in speicific circumstances like debugging. For starter I would recommend to avoid that.

Good luck.

/Q

...and start by learning how to be a decent editor first. It will save time in the long run to have a good general knowledge of what EPiServer is actually used for. Play around with alloy site and add images, text, pages, blocks, language versions etc. That's the best start of becoming a good developer for EPiServer. Welcome to the best CMS :)