Well for a start if your learning CFD, start with basic models, maybe even 2d models. You need to get an understanding of the program at a basic level first. If you complicate your models you complicate your meshing process and that is perhaps the most crucial stage of CFD pre-processing.
What is your understanding of fluid flows like? Are you a student learning? If you have no understanding or even limited understanding then it is most likley all you will be doing is producing pretty pictures.
EDIT - just read your other post. You are a final year student. With that in mind, stick to the advice of starting simple and working your way up. I started learning CFD last year. Having used (or tried) star CCM, openFOAM, and the newer ANSYS (dont know what their called), I muched prefered the older Fluent (5/6) and Gambit as a pre-processing program. Lots of tutorials about and easyish to pick up. What program is it you'll be using?