With the battery. Are you talking about a regular incandescent 100W (double-u, not vee) bulb? Just hook up the battery to the bulb -- one wire to the bottom, and one wire to the screw-in side. A 100W bulb will draw 8.5A from a 12V battery, so the battery will drain pretty quickly. If you're trying to use a CFL or something, you'll need a power inverter as discussed elsewhere.
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If it is a household bulb, then use an inverter. You can find them at most hardware stores, such as Home Depot or Canadian tire.