Determine the element's period, keeping in mind that while periods are the name scientists give to the vertical columns of the table, they are not necessarily numbered sequentially. The middle section of the table -- the transition metals -- constitutes a secondary, or "B," group of periods. In order from one to eight, the "A" group of periods feature the following elements atop them: H; Be; B; C; N; O; F; Ne. Phosphorous (P), for example, is in the fifth period, or period "VA."
Use the element's period to determine how many valence electrons it has. The correlation is direct. Phosphorous, for example, has five valence electrons because it sits in period VA.
Recognize that for elements in "A" periods, two exceptions to this rule exist: helium (He), which has two valence electrons despite sitting atop period VIIIA; and the rest of the elements in period VIIIA, the inert "noble gases," which have zero valence electrons.