We are only after the direct reflections. I use a Rane . An RTA is a device used to give a readout of the amplitude of different frequencies. Little flickering lights will tell you which frequencies are stronger and which are weaker. RTA's also usually come with a built in sine wave, or white noise feature which will provide a good, flat frequency range to use as a guide when setting up a sound system.
Most units come with a small microphone to use for input.
I like to use the white noise through a mixer channel and aim the little mike around and study the differences in the room. Try this with music too. Look for large differences in the lows in corners and where the floor meets a wall. Some monitors come with built in amplifiers like the Mackie HR824's and some require the purchase of an outboard amplifier like the Yamaha NS10MS . Either choice is good as long as you don't cheap out on the amp.