How’s it going? Starting with CAD and Simulation might feel like it’s too much for sure. Your use case is a very common CFD application. However, we can only provide you with tips on your simulation setup for you to be able to do it yourself!
For flow simulations, it would be nice to start with some of our incompressible solver tutorials. These are particularly about internal flows (I understand this is what you want to achieve).
Hint: The first step would be to create a flow region using your duct geometry. You can do this after importing the CAD into SimScale. After that, you can setup a flow simulation (just like in the above tutorials) on the created flow region.