So you've decided to acquire some crystals of your own, but you're not quite sure how to go about buying one. What should you look for? Where should you go? How do you know which crystal is the right one for you?
The first step is to provide yourself plenty of access to a good crystal piece. I would recommend buying from someplace where you can actually be present, rather than ordering something online or through a catalog. This way, you are free to pick and choose from a selection, rather than have an employee of the shop choose one for you. New Age shops, fairs, flea markets, and bookstores are all good choices of places to begin seeking your crystal collection.
Once there, give yourself some time. Pick up crystals, touch and feel them until you can sense which crystal seems to "feel" right to you. Remember to think about why you're selecting a crystal, consider the purpose for this acquisition. When you feel the right one, that's the one you want! Remember, just as you're choosing the crystal, the crystal is also choosing you based on your vibrations.
Some people who are fatalists also believe that when a crystal is offered to you by someone else, take it -- it will be more powerful as it has been chosen by someone else. Both schools of thought are fine; do whatever you feel most comfortable with. I would prefer to choose my own, but if someone gives me a crystal, I would be happy to accept it as well -- both methods can have their uses. If you're not able to go somewhere to choose your crystal personally, you can still enjoy one which has been purchased online or given to you by someone else.
After aquiring your crystal, the first thing you need to do is to clear any vibrations or energies that it may have absorbed before it has come to you. My family's traditional practices for this was to soak our crystals in a glass of salt water for a few days, which can cleanse and purify any unwanted energy. However, in recent times I've also heard that crystals can be purified by simply running it under some flowing water for a minute or two. Use a swimming pool, a stream, or even your faucet -- visualize the water flowing through the crystal, pushing all of the old energy out. Your crystal will become purified in this manner; you will intuitively feel when the crystal has been cleansed adequately.
Once it has been cleansed, it is time to "program" your crystal with your own specific energy signature. The easiest method of doing this is like so: Put the crystal in your "sending hand" -- usually, this is your right hand, though it can also be your left hand if you're a lefty -- and imagine a white light shining above your body (on your crown chakra), then travelling downward, entering your body and shining all around you. Visualize the light moving down your arms, to your hand, and then filling the crystal with your spiritual energy. Once you are satisfied that the energy has filled your crystal, it is ready.
Strengthening the power of your crystal is optional, but can help if you feel that it needs a little extra boost from time to time. There are a number of ways to do this... You can try burying it in the ground for a day or two, allowing it to absorb the energy of the Earth and strengthen its power. Or, leave it outside in the sunlight for a few hours to aborb solar energy. Some people leave it out only at night to absorb lunar and starlight power as well -- this is the tradition in my own family. Any of these will provide varying levels of efficacy, just do what feels right for yourself, the crystal you're working with, and the situation at hand.