I have taken measurements of a phenonenom via two different methods. I
want to know if the methods can be interchanged: can I measure using
one method only and make assumptions about what the value from the
other method would be. I took all of the pairs of measurements and
created an x-y scatter plot in Excel. The correlation was poor
(not surprising) and appears to be linear. I had excel calculate and draw
the linear correlation and it gives me the r and r-squared values,
which again are low. I am being asked what the p value is - how likely
is my result to be valid. Everything on the web regarding p-values
makes sense to me only up through the definition of the term. After
that I get lost.  There are plenty of stats programs for excel I can
download, but even based on their demos and descriptions, I'm not sure
how to proceed.

Details: I am measuring flow. Using a single instrument, I measured
peak flow and volume for each event. I have 538 samples. I want to
know if peak flow is a valid surrogate for volume. I have no reference
for how these two measurements should correlate. Using XY scatter plot
assuming a linear correlation, Excel shows r-squared = 0.325. How
confident am I in this result - what is my p-value? How do I calculate
it? Don't ask for clarification if I want a one tailed or two tailed
answer, etc. These are the exact questions I can't give an educated
answer to.

Thanks