Learn normalisation from wikipedia, then read "The Art of SQL" to get a feeling for the shape of the weirdnesses, then read the postgresql documentation from end to end.

(disclaimer: worked for me, may not for anybody else)

I intend to read the PostgreSQL docs end to end at some point, but last time I checked the full PDF comes out to something like 3600 pages! I am astounded by the quality of everything about PostgreSQL, the community included.