Investigate a product called Room EQ Wizard. It's a free program for the PC that's MUCH easier than taking individual measurements with a meter. You need a mic and a full duplex soundcard. For starters, you can even use the meter's built in mic.
Measuring before treatment is more for placement of speakers, sub, and seating. After that, you're wanting to look more at the remaining peaks and nulls, identify them based on what's causing them, and address those issues. Also, you need to look at overall decay time across the spectrum. Those will be addressed in most cases by broadband bass control and a series of reflection point panels.
Bryan