middle pedal? how to use it, actually? 
with no pedal depressed, pressing a key causes the damper of that note to lift, and stay lifted until the key is released. the hammer strikes the note shortly after the damper rises, creating a sound which continues until the note is released, when it stops.
left (una corda, "quiet") pedal uses various mechanisms to make everything less loud. in doing so, the timbre (sound quality) is changed. dampers are unaffected.
middle (sostenuto) pedal holds up the dampers on any keys which were pressed at the time it was depressed, and leaves all the other dampers on. this allows those notes to continue to ring while still letting you play other notes staccato without muddying the waters.
some uprights don't have a sostenuto pedal at all (only 2 pedals), and others have a practice pedal as the middle pedal, which is similar to the una corda pedal in that it greatly quiets the sound but changes the timbre (poorly in most people's estimation).
right (sustain, "loud") pedal lifts up all the dampers, allowing any note to continue to ring for as long as the pedal is pressed. leads to muddiness if left too long, but essential for a number of different things.
as for how to use the pedals...there's whole books written on that, and I can't claim to be a pedal use expert.