Can can tell me how to use the ZXing library in an android app in TITANIUM.I have downloaded the ZXing library from the following path : http://code.google.com/p/zxing/downloads/detail?name=ZXing-2.0.zip
Creating a custom module is the only way to do this. Therefore you have to have knowledge of the native platform.
Modules are not too difficult if you have native platform experience (or just know Java for Android). I would check the moddevguide example module which is chock full of examples of passing data back and forth, which is available in the public Titanium repository on github. Also, the paint module is a great example of subclassing to roll your own titanium component.
So first go here and follow these step-by-step instructions on how to start your own module. Documentation Link
Then check out this code for great examples of passing data throughout your app. Titanium Github
All that aside, if you dont have native platform experience, why not just use either the Titanium Plus modules barcode scanner? Or this open source alternative?
No need to reinvent the wheel at this point.