A good book to use for deciding what to listen to is The Record Shelf by Jim Svejda. It has many editions; but any would work. Svejda lists recordings by composer and recommends his preference as to recording artists and as to the composer. He is informed and succinct and fun to read. He's not the final authority, but his book is a good place to start.
Another good book is Classical Music for Beginners. It is in comic book format.
Basically, any musician with decent training should know about the four major periods of music history: Baroque, Classical, Romantic, and Contemporary, and should have a sense of what each period is like, and should be able to name four or five famous composers from each period.
Once you know that stuff, then you should start to branch out and learn more periods (Ars Nova, Renaissance, Impressionism, Empfindasmer Stil, galant, etc.).
The whole point is that you get the tools you need to be able to CARE about all this stuff. It's all for the sake of you liking music more, so you should approach it as a treasure hunt in which there is all this free treasure for the taking, and there is so much you can't take it all, so you have to pick and choose which treasure you think is the prettiest! Not because you're going to go sell the treasure to someone else; but because you want to look at the treasure for yourself.
Have fun! I envy someone who is about to discover the bounty of classical music: you're in for a big treat!
Topic: Where To Start...