In terms of correspondence, "to will" is generally correlated with fire and passion or desire.
Making spirituality an integrated part of life is not easy. This is not a "go to church on Sundays, but do what you want the rest of time" sort of religion. This is something you take with you all the time, something that colors your perceptions and your interpretations of what you encounter on a daily basis. It takes strength. It takes determination. It takes power of will. To will is not just to want something, it is making a commitment to a plan of action and following through to create the change we desire. In this case, that "plan of action" is magick.
It also has something to do with the concept of "to know," in the sense of planning. Performing magick on a whim, without planning ahead or preparing properly, indicates a lack of commitment to the change you wish to make. Whether you believe in a conscious deity or simply a universe full of energy, the result of "sloppy magick" can be the same - a lesson learned the hard way. Sometimes we need the lesson, but sometimes it could have been avoided by focusing our wills toward accomplishing our goals.
The will is also the part of you that actively manipulated energy to perform magick by causing change. It must be steadfast and confident in order to be effective. If you are unsure about any aspect of your magick, you will see the effects in your results (or, lack thereof). If you are unsure of yourself, your magick will reflect it. The easiest way to assure a strong will is to ensure that you are abiding by your own sense of morals and ethics. For some, this is the "Harm none" rule, or the "Three-fold law." For me, it's the Golden Rule - "Do unto others as you would have them do unto you." I end every spell and chant with "And it harm none, so mote it be" to reflect the purity of my intentions and my desire to prevent harm from befalling anyone (or anything). Harm can't be completely avoided, but in a system where intention matters, simply endeavoring to prevent harm can be enough to minimize it as much as possible.
So what is "to will?"
It is to commit to your religion and spirituality.
It is to commit to your magick and your goals by planning ahead and following through.
It is to perform magick confidently, being true to yourself and your own sense of "right and wrong."
"Always bear in mind that your own resolution to succeed is more important than any one thing."
-- Abraham Lincoln (1809-1865)