This can be either very hard, or very easy. Choosing a crystal (and I'm talking quartz here) is a personal thing. They have to feel right for you. This is something that really can't be done on line or over the phone. You need to be able to put that crystal in your hand and FEEL it.
Going to a gem and jewelry show, gem and mineral shows, or psychic fair are good places to find crystals of all shapes and sizes.
When you do approach the crystals, it seems as if you can feel them say "hey!! take me home!! yes, me!! over here!!" but you have to touch them to know they are right for you. First impressions are the best. If you feel it's right, and you keep coming back to that one crystal, then it's meant to be yours. It doesn't have to be the biggest or flashiest. I've seen some ugly ass crystals in my time that have been some of the most powerful for a person.
Once you take it home you'll want to cleanse it and give it purpose. Meditation works here for that. Even just holding it and visualizing what you want to achieve works. It doesn't have to be a long drawn out ritual.
Next topic will be working with your crystals. and some of what other gems can do for you.