Solidworks is what I regularly use and I got the student version for only 4 euros at school
(Note though that normally student versions are up to 100 euros). It is an absolutely amazing piece of software! The freedom of shapes you can create is big but you do need some experience to create the more complex parts. However, it is well worth it: You can create parts very accurately and there are lots of ways to cleverly build your components with the right tools. When you're done building you can then put it through simulations to put your design to the test even before you have built it!
Only drawback for you: You need windows to run it.
So if you want to get into designing in CAD very seriously and intend to use the models to actually build stuff, Solidworks will suit you. What I'd recommend you to do is go to a local university, get in contact with them about buying a license from them or finding a student to help you out, so to speak
However, if you don't plan to use the software a lot, I suggest you get something more simple. For things like plate parts you could even use software for vectors like Corel Draw and Adobe Illustrator. These can save .DWG and .DXF files that are often used for making plate parts and/or engraving flat surfaces.
If you just want a piece of software to make shapes and go for aesthetics, you could also go for Rhino 3D. There is a Beta version for OSX, and I believe that is free
The version for windows is a trial, and buying the software costs about 1000USD.
And then finally, if you need only one part made or occasionally, you could ask someone to make the CAD file for you - just PM me if you're interested in that.