To find out when the next full moon will be, use an online directory of full moon dates or make an educated prediction based upon the moon's current place in its cycle. Some online directories of moon dates include MoonGiant.com, MoonPhases.info and Space.com.
The moon travels around the Earth every 29.5 days, meaning the next full moon is 29.5 days from the last. Contrary to common misconceptions, the phases of the moon are not caused by the earth's shadow, but rather the fact that half of the moon is always in shadow while half of it is always lit by the sun, just as with the Earth. As the Moon revolves around the Earth, different portions of that shadow are visible, going from new moon to half moon to full moon and back.
MoonGiant.com and MoonPhases.info are websites specifically dedicated to the moon and both list its cycle dates. MoonGiant.com also lists other events, such as solar and lunar eclipses. Space.com lists full moon dates and their traditional names, such as Wolf Moon, Strawberry Moon and Harvest Moon.
To approximately guess the cycle of the moon, look at the side lit by the sun. When the moon is lit on the left side and in shadow on the right, it is a waning moon. When lit on the right, the moon is waxing, and the next full moon is no more than 14 days away.