You just make one up. If you want to find out about it in a more meaningful way, then try meditating and picking the first words that come to your mind and write them down, then choose which ones have speacial meanings to you.
Some traditions recognize practitioners having more than one magickal name. One being a publicly known name and the other being your private or secret name given to you by the Goddess and God. Your publicly known Craft name should be one that you feel describes you best. This can be one based on legend and/ or nature. Whatever name you choose, think on it for some time before you finalize your decision. You don't want to start confusing your friends and associates by constantly changing your name. To find your public Craft name, try my worksheet.
Your private magickal name is a different story all together. This name will come to you. It may be months or years, or it may come in a couple of days. This name will be given to you usually in a dream you have or during deep meditation. One thing is for sure, when it's given to you, you'll know it! It is said that this name, given to you by the Goddess and God, is your true and right name. But it is never to be spoken aloud or to be shared with anyone save the Goddess and God. It is believed by some that for another to know your true right name gives that person power over you.
You can use meditation to help you search out your secret name. It may take several tries before this name is revealed to you. If you don't have success the first time you try, don't push it! Remember the name comes to you when the Goddess and God feel you are ready to receive it.
Your name should simply come to u when ur ready to recieve it. It may take a while, but u'll know once u find it. Sometimes it can be an anagram for ur non-magickal name. Other times it takes months of research & pondering. I didn't learn mine until 4 months after I discovered I was a witch, but everyone is different. Just be paitent, it should come soon.