Reaper will be a great program for you for making your own music. Adding a keyboard is another step. I am assuming you are talking about a piano style keyboard, and not a typing keyboard, so there are options to go with for that.
A lot of DAWS have built in Midi plugins called Virtual Instruments or VSTi pluigins. VSTi plugins come in a variety of options from pianos, to organs, to string instruments, and drum kits. Midi keyboards can trigger the sounds of the VSTi pliugins, and you can record the midi data in Reaper, then rendering it down into a WAVE or MP3 file to add to your videos.
However, if you are not familiar with VSTi plugs, midi keyboards, or audio interfaces with Midi inputs/outputs, you are going to want to get a lot more educated on them before attempting to use them. You are opening a whole other can of worms as far as options in interfaces and what not go, and they all have needed drivers and appropriate settings to go with them based on your PC hardware.
Ya we never really got that far into the program with those, mainly film and such. I'm working in broadcast but I still want to expand on the area of which why I went to school. I mean knowing more never hurt anyone. I used audacity previously to record voice and such bit I found it alittle basic