Mathematica help (please!)

I'm working on some data atm, and decided to use Mathematica because I've some experience with it. Sadly, it turns out my skills in it are currently limited to calculation-style problems (using it to check assignment answers!), and I'd quite like to get the hang of it for other uses!

So I'll get to the problem. I've imported several text files of data into Mathematica, and I'm trying to plot data sets on one graph. I want a log plot as there's exponential fall-off, but I'm not entirely sure that I'm going about it in the right way. I'm trying to use ListLogPlot as per the way it's explained in the help section, but I just get an empty graph. Can anyone advise? I suspect that using gnuplot might be somewhat easier (and will resort to it if necessary), but I'd like to get this!