Could you please tell me why there isnt a new software update for the iphone 3g latest being 4.2.1?
Because the hardware in the iPhone 3G is insufficient to run the newer features of the newer versions of iOS.
Can anyone tell me if i can do anything to get better software
If you mean iOS, you can't. iOS 4.2.1 is the last version your iPhone will run.
or how i can use the apps?
You'll need to find apps that the developers have kept compatible with older versions of iOS and older iPhone hardware. Or buy a new, or newer, iPhone.
Is there a software update due?
The chances that there will be any further updates to iOS for the iPhone 3G are vanishingly small.