Should I use the absolute value of a correlation coefficient?

I have calculated the Pearson correlation coefficient for two sets of values using excel. I got the value in the negative number. I know that the idea of getting a negative correlation means that the two variables are reversely proportional. Now, I have to calculate multiple/joint correlation between other variables. Should I use only the absolute value of the first correlation for that case? Or should I use the exact value with the sign? Please guide.