linear motion sensor- help

hey,
I want to write a program that do something whenever it detects a change in movement... (such as moving up and than moving down).
I'm sure I will need to use a motion sensor for it..
could you help me about the steps I sould do??
how will the software programming part integrate with the sensor??

Thanks!!

I'd start with an Arduino and learning how to use that.
After that, grab an accelerometer and learn how to talk to it with the Arduino.
After that, link it up to whatever software you're after.

Since this is in the audio section, you could play an ultrasonic pulse and then listen for its echo, correlating the delay to the distance to the object.
http://stevetarzia.com/sonar/