Unlocked iPhone 3gs cant activate iMessage

iMessage doesn't work when sending to non-iOS devices. It is an Apple service that only works with other Apple devices. And T-Mobile is not a supported carrier, so some features such as MMS are not available.

But how can i activate it to merge with my number?

You cannot send iMessages to non-Apple devices. Doesn't matter if it's your number or your email address.

I understand but in messages settings it says to activate iMessage and i want to it'll give me un successful activation , to be able to text my girlfriend who has a 4s. any ideas?

Check T-Mobile's user forum, you can get some pointers there for iMessage, MMS, Facetime, etc.