Pick a crystal
Only one? What if I take a load? Well, crystals are very handy. For example, you can throw them at people who are very annoying (just kidding, or am I? Duck! :)
A very interesting fact is that a snowflake is actually also a crystal, made of ice, and won't last long in a living room or on an altar, but still a crystal. If you look at pictures (zoomed in), they are actually very beautiful.
A crystal is a solid material formed in the earth or, like a snowflake, by solidification. In my opinion, they take the power of the earth they are in, and that is why every single one of them has powers, from very soft healing or comforting powers to banishing people/entities. The powers are connected to the materials they are formed from, the earth they are formed in, and the time process that gives the crystals power.
It is important to keep your crystals clean and charged. The best way to charge them is under a full moon, but if you need to use them right away with no full moon in sight, you can use running water to cleanse them. Crystals can be used in any magic work, from protection to shielding, from making you happier to healing your body. Many pagans wear them every day. It is important that you don't just wear everything together. If you read the forums about witchcraft, you see a lot of questions about what to do with broken crystals. It does break for a reason. If it breaks or you lose it, it served its purpose. Don't try to mend it with, for instance, wire, or if you do, don't expect it to be of any use for the meaning it once had.
Magic work with crystals
Wear them (pick your stone depending on what you need and simply put them in your pocket or bra)
Use them with work on people (healing, cleansing, lifting up spirit)
Use them with work on the land (healing, removing negative energy, giving energy)
Use them for meditations (for more focus/better visions/support)
Chakra opening/healing (place them on your chakras)
To assist you with oracle work like tarot, ogham, or runes
Charge your altar with crystals
Channel energy
Sacrifice them to the land (for instance, as part of a spell or ritual)
When I look at my favorite crystals, there have always been a few that stand out for me and help me a lot with my work.