Assuming you are using a full-size, standard 88 note piano, middle C is the fourth C reading from the left (bass notes). Hence it is often referred to as "C4".
If you are referring to Middle C on an external MIDI keyboard controller then you may have to consult the keyboard manual.
MIDI keyboard manufacturers are not required to put middle C in a 'standard' position as in this article for the various sizes of keyboard.
Another issue you may have is with octave changes, which are often required for the smaller MIDI keyboards especially those that have 25 keys - notes that you want to play may not be available as they have moved off the keyboard.