Well ... really, just read a good pair of textbooks on each side of the spectrum. Date's Databases and Kimball's The Data Warehouse Toolkit are good.
Edit: actually, maybe not Date. It's up to you. It's good, but it's controversial because he's not a fan of SQL and so he uses his own language.
The one I used in uni was Ramakrishnan & Gehrke's Database Management. It was OK but there's a certain amount of at-the-time trendy bullshit that to me detracts from a focus on relational databases for their own sake.
Edit 2: and Joe Celko's SQL for Smarties contains good oil on the relational paradigm.
