There's so much to crystal magick that it's best to just read up on it. A good website to check out is www.healingcrystals.com. Also you kind of have to learn on your own because you have to feel them in order to see which ones will work best for you.
There is a really nice article on the topic here on the site as well. If you do a quick google search, you'll also find a lot of good info. I added the link below, just copy and paste it into your browser bar and remove any spaces that appear; then it should work just fine.
I personally would recommend the books Crystal Healing and The Crystal Bible by Judy Hall. Both have really lovely information on crystals, their properties, uses, etc.
There are also some fantastic resource books out there for referencing a whole host of crystals and what they do. 'The crystal bible' is a very popular one. It can be found online and in most metaphysical/magic shops. it even comes in two different versions; a compressed 'pocket' book that is about five inches square, and a full-sized tome that has a wider selection of crystals, larger pictures, and a smidgeon more detail.
A good idea, besides reading up and all, is looking at what crystals call you most and start reading about those. If you have a bunch of crystals already, learn to work with those and work from there! Doing the theoretic reading on them and actually working with them at the same time can make it better for you.
Besides, there might be correspondences that might be particular to you (for example, you might relate rose quartz with knowledge and mentorship because your grandma was a wise image to you and she always wore a rose quartz pendant, just an example here but you get the idea). Some people have specific correspondences for crystals particular to them because they worked with them and realised they worked for something that wasn't initially in the books. So, practicing with them is also good to start!