How do I properly receive MIDI with Arduino, with 6n138?
Most start at the midi site at playground.arduino.cc.  But there are numerous web sites across the Internet.  It is important to understand that the MIDI interface uses a current loop to send and receive data.  While the Arduino normally uses voltage levels to communicate.  As such, an electronic interface needs to be built or bought to convert between these two types of signals.  Some interface designs are simple and limited.  Others are complex and robust.