Before you even attempt this ritual, you should have MASTERED visualization, and/or putting yourself into a trance or alpha state. The moon symbolizes the Goddess, therefore, calling down the moon is essentially calling down the Goddess, and allowing her essence to enter your body; it is ritual possession, religious ecstasy. Depending on your state of mind, and ability to trance and visualize, it can be the most powerfully spiritual experience of your life, or the most disappointing; just as taking communion to some Catholics is consuming the body of their savior and to others, it's just eating a dry wafer.
There are many elegant and inspiring calls to the Goddess available in books and on the web as well if you need them; I could write another, but your own heartfelt words are always the best. There are also dozens of full blown rituals to choose from out there, so I'll stick with the bare bones.
This ritual should be done on the full moon and, if at all possible, outside. If you can't do it outside, do it in a room where you can see the full moon clearly through a window and let its light shine down on you. This is important, this isn't just another Esbat. Carefully and thoroughly cleanse and purify your ritual area and set up as much as you can ahead of time, even a day ahead if you can, so you can make those last minute adjustments and additions without distracting yourself or breaking your concentration once your preparations have begun.
Make sure that on the night of this ritual you absolutely will not be interrupted.
Take a long cleansing bath, listen to appropriate music to begin to get you into the right state of mind, meditate while you bathe, clear out the mundane "garbage" cluttering your mind, bring in purity and light.
After your bath, anoint yourself with Full Moon Oil, refer to any good Wiccan text for simple ingredients and instructions. I prefer to do this rite skyclad, but if you wear a robe, make sure it is clean. I strongly suggest that you wear no jewelry for this ritual; present yourself to your Goddess with simple dignity.
Cast your circle with extra care. Before you raise your cone of power in whatever fashion you choose, smudge yourself within the protection of your circle. Face the moon. Now is the time to say whatever invocation you have decided upon. Either repeat it, or a chant, raising your arms to the moon and dropping into a trance state.
Visualize the moon growing larger and larger, nearer and nearer as you chant or say your invocation, until you begin to feel the cool wash of divine power flow into you and fill you up. The key to the success of this rite, is to freely and completely entrust yourself to the Goddess, drop all barriers, and allow her to possess you.
If this is your first time doing the ritual, I suggest you stand with your eyes closed and commune with the Goddess, understand her power and love for you, and that a part of you is always within you. You will sense when it's time to break the communion and allow the power to gently withdraw from you.
Thank the Goddess, and complete your ritual as usual. MAKE SURE YOU GROUND AND CENTER! Don't just partake of your feast, in addition to eating, put your hands on the ground palm down and release excess power into the Earth to help her heal. Don't worry, you'll still be more than energized enough.