Looks like you need a simple audio interface and a microphone.
In my opinion (and i'm sure the vast majority of people thinks the same) it sounds way better if you record your acoustic guitar with a microphone. You could always blend the direct signal and the microphone signal together if you wanted to record both.
What kind of electric piano do you have? MIDI is probably the way to go with, but you would then need a virtual instrument plugin in your DAW. (Digital Audio Workstation).
This may be a bit overwhelming so i would suggest to Google ''12 home studio necessities''. The articles can be found both in text and video. Enjoy!
Hope this helps.
-T
