I had an iphone and lost it so i bought an BB Pearl 8120 from the bay.
I am okay to use the internet browser and download such apps like Gmail, but when i launch the app a msg comes up saying that this programme requires a working data..

Is it any thing i need to change on my BB to make it work?

Please some one help me..

Probably trying to tell you that you need a BB data plan. Get that and most of the problems go away and the workarounds aren't needed.

From the FAQ:

How to use BlackBerry on Generic Data Plan - BlackBerryFAQ

What do I need a Data Plan for? - BlackBerryFAQ