newbie needing help with getting dddl 8 or later software

hello all you gd people im a total newbie to these download type software stuff i look after a fleet of 9 detroits for my fathers buisness on my own so as you can imaigine my time is spread very thin so being able to diagnose things with the laptop will be a great help to me so if sombody could point me in the right direction in regards to what info you guys need to help me an what i need to do to make this thing work an talk to these late model detroits would be very much apreciated thank you in advance

Do you have a Diagnostic Link?  I use a Cummins Inline 6 (Serial Port) and a Inline 7 (Wifi-Bluetooth).  And what year models are your dads trucks?