If you are planning to use Reaper then you will have tutorials on YouTube giving you step by step instructions from the very basics to advanced recording techniques so you shouldn't have much trouble getting started using that DAW.

Impulse responses are simulations of guitar cabinets and speakers that sound pretty convincing, if you are recording guitar straight from your boss katana you would load an impulse response onto that track to emulate the sound of the amp speaker being mic'd up. The bonus of recording ideas this way is you can alter the guitar sound very easily by changing the speaker or mic distance of the impulse response.

I use pulse to load the impulse responses into reaper, it comes with one can installed which is pretty good but I've got a version of a Zilla cab loaded with V30's I use most of the time.

You can download pulse for free from this link.

https://lancasteraudio.com/pulse/