i've tried using the du articulation but i cant seem to get the puffy type of articulation that Delfeayo Marsalis has
I currently use a Getzen trombone 351 series and a Getzen 12c mouthpiece
please help

First of all, you should invest in a better/bigger mouthpiece. Look for a 6 1/2 AL or something similar. That should help

Instead of using "du", use "dah". This should help you open up your sound and give your articulations more definition.

Also, if you're just beginning as a trombone player, a 12C should be fine. But as you progress, you might want to look into something bigger. A 12C is quite small for most trombone players nowadays, even players who play lead parts in jazz bands.