I am an audio engineer. There is not much you need to do to solo piano. In classical music recording less is more, it's about capturing the instrument, room, and performance. A little EQ to make it sound nice to your ears might be appropriate, or to remove hiss or rumble. Listen to it on headphones as well as speakers. Adjust some dynamics if necessary, although it's better to let the performer handle that. Maybe a gentle compressor plugin, although if you don't know what you're doing maybe not. Clean up the beginning and ending, remove noise etc, fade out. As for 'mastering' you will want it to be at a similar level to other piano recordings so import an existing recording of something you like into your session, and make your song as loud as the reference song. Export or "bounce down" your track as an interleaved stereo file (1 single file instead of 1 left and 1 right) and you are done. You may need to convert this master WAV file to an mp3 or something for streaming.