I am an absolute beginner when it comes to 3d printing. I want to get into the hobby by designing aero automotive parts such as fender flares, custom gauge and switch pods, lips etc. I've never taken a CAD course but I would say I'm proficient enough with computers as I work as a developer. My uncle is an architect and it seems like autocad might be something good to go with. What are some other good alternatives that allow accurate modeling down to millimeters and possibly breaking larger objects into smaller 3d printable pieces to mash together?
You've avoided a number of attributes of a poor question by specifying your objective in detail, while also providing some indication of your experience. AutoCAD is not well known as being 3d printer friendly, although a skilled AutoCAD design user may be able to create acceptable models.
One could consider more organic modeling software such as Blender, although it works in reverse of my comment above. It's great for free-form model building but not so much for engineering type construction. Again, a competent Blender user can avail himself of the parametric aspects of the software, but it's more work than one would necessarily desire to engage.
SolidWorks has a number of followers in the 3d modeling community. My experience with the software has shown me that it can be learned fairly quickly and is powerful enough to allow one to explode creatively over time.
The structure of Solidworks workflow fits in directly with your description, both to the reference to accurate modeling as well as the break-apart aspect. The construction of a model in SW is done via sketches and various actions performed on resultant objects created from those sketches. It's not free software, rather pricey, but it may also save you so much work as to pay for itself over time.
There are other options, of course, some of which are free to students and hobbyists. I did a quick Google search for "free parametric 3d modeling software" which resulted in this link:
You will find many suitable programs in the list generated from that link, as well as many that are unsuitable for the reasons mentioned previously.
Consider also to view YouTube videos of your candidate selections. Most of the programs referenced will have tutorial videos which can give you a clearer idea of how well it fits your needs and perhaps how easily it can be understood.