The concept of the Maiden/Mother/Crone describes the Goddess in Wicca as representing all three phases of a woman's life. You don't pick just one aspect to work with, the idea is to work with all three in balance throughout the cycle of the Wheel of the Year. The Maiden/Mother/Crone are aspects of the Wiccan Goddess related to the Moon.
As an aside, the concept of Maiden/Mother/Crone is a modern one first spoken of in the book "The White Goddess" by Robert Graves. Although there are triple Goddesses in several different mythologies it would be wrong to try to fit them into the Maiden/Mother/Crone mould as the ancients never vewied their Deities in such a way. Nor should one try to assign one of the aspects to an ancient Goddess as it is a thoroughly modern way of viewing Deity.
Your zodiac sign and the Element associated with that sign have absolutely no relavence to which Deities you find yourself eventually working with. Nor does the Element attached to your zodiac sign assign any such Elemental association to yourself. You are made up of all of the Elements and should be working with each of them in order to be in balance.
If I might suggest a couple of reliable books to read in order to get a better understanding of Wiccan beliefs I would suggest:
"Wicca: A Year and a Day" by Timothy Roderick
"Wicca For Beginners" by Thea Sabin
And a reliable website can be found at http://wicca.cnbeyer.com/