trendlines

I am trying to obtain a expression of a data set that i know is exponential on a linear plot.
I have plotted the data on a log scale, to give me a linear relationship, i want to find the equation of the straight line on the log plot.
Everytime i do this it flags a fault, saying that i either have to plot positive numbers (which i have) or change the scale which i do not want to do.
Please help!!!!!!