According to The Cartoon Guide to Statistics, pvalue is a probability statement which answers the question: If the Null Hypothesis is true, then what is the probability of observing test statistics at least as extreme as the one observed. The Null Hypothesis, is usually that the observations are the result of chance. The Alternative Hypothesis is that there is a real effect, that the observations are the result of this real effect, plus chance variation.
Things You'll Need
 Statistical Software

State the Null Hypothesis and the alternative to it.

Select the level of significance.

Select the test statistic.

Determine the appropriate sampling distribution for the test statistic.

Find the sample value of the test statistic and accept the null hypothesis if the value of the test statistic lies within the acceptance region. You now have the pvalue.
References
 Statistics in a Nutshell, Sarah Boslaugh, Paul Andrew Watters, 2008
 The Cartoon Guide To Statistics, Laryy Gonick, Woolcott Smith, 1993
 Basic Statistics: An Introduction, George Summers, William Peters, Charles Armstring, 1977, p 218