reset arduino

Hello everyone, i'm new with arduino's technology. i'm a french beginner, soory to ask for question that u can find very easy.
i can't find any information to reset my arduino nano. By resseting i mean, empty sketchs in the memory...
Have u got an answer?
is there a way to know available memory?

Just load an empty sketch.
Why do you want to erase the sketch at all?

http://playground.arduino.cc/Code/AvailableMemory

Just load an empty sketch.
You mean it will erase the other(s) sketch?
moreover, is it possible to have 2 sketchs in the memory? Or we can just put one sketch for one arduino?
I want to upload the sketch "BinarySwitchSensor by MySensors "& "DistanceSensor by MySensors ", is it possible?

No, you can only have one sketch at a time in an Arduino.

I may be possible to melt two sketches into one,
but that depends, and has to be done by hand.

when i go to "Arduino Playground - AvailableMemory"
i find 2 programs (or libraries i dont know the real name) that are named "MemoryFree.h" & "MemoryFree.cpp" and one exemple. I dont know what to do? Save the 2 programs somewhere (i don't know where) and run the exemple to know the availaible memory?

Thx u for your answers Whandall.

Maybe they were a little terse, sorry for that.