How to find when a customer first started doing business with you in SQL

Hi, 

I am to find when a customer first started doing business with my company in the MySQL db. I am pretty new to sugar's database I figured I would ask on here to save some time and frustration.  Are there any resources for figuring out how the database's relationships work?  I noticed that each modules have about 5 tables and they're used to relate to other tables but I am not completely clear on how everything is set up. If anyone can shed some light on this it would be very appreciated.

Thanks