Some questions about accelerometer

I want to know that I want use the phone if it shaking or not to decide the next step , how can i white it ?

just like if shaking do A else do B. 

This explains how to use the  Accelerometer  http://appinventor.mit.edu/explore/sites/all/files/hourofcode/TalkToMePart2.pdf 

You will also need an IF..then statement somewhere, depending on your code.   There is no  AccelerometerSensor.Shaking true or false block, so you will have to try some things.