I use Rhino, and what I like about it, is that command help shows up as soon as you click on a command. Though I admit that I have Rhino primarily because the Mac version (only) was sale priced last year.
But no matter what software you get, you should acquaint yourself with the progression of drawing objects and formats. In short, you start with 2D & 3D objects, surfaces, and solids. You should keep copies of the file as you build up your design, because as you approach generating a Stereo Lithography format (STL) for importing into a slicer program, you need to convert your complete design into one or more watertight meshes. But meshes are painfully difficult to fix if they have any “manifold” problems, so as you attempt to create your mesh, and it has problems, you’ll want to go back to a version of the file that is pre-mesh. If this is a big vocabulary lesson, it is. And I’m just getting started, and no expert, but I have already tried drawing a relatively simple design, got to the STL and found issues that I couldn’t fix because I did not keep any intermediate versions of the drawing.
Tinkercad is easy, free, and fun. Nothing to install and it does most of what the average person would need. I have zero experience but tinkercad and YouTube made it easy.
Rhino is great, but you’ll realize that is actually incredibly weak; complex models will often require alot of post production stl work. You find your self mostly using the perspective view frame which makes accuracy horrid.
It is simple to use and pick up, but by fair its best quality is that options in exporting files. Plus the plugins (grasshopper, etc)
I can really recommend Autodesk Inventor. I am using version 2014 since I only got a 32 bit system but you can even get the latest version for free as a student 
As some people mentioned before, you can even try Fusion 360 but for that you need a 64 bit system.
With the help of the tutorials and other youtube videos you will get along with that pretty soon 
I love openscad, it’s is a scripting language and it seems perfect for my use…
Philippe
FreeCAD! I find the constraint-based sketch editor to be extremely handy. I really do wish that composition of multiple 3D shapes was more powerful, but this is still an excellent tool even for fairly complex parts.
I did notice that there was a lot of talk about programs, but little about hardware.
The requirements for a PC will vary depending on how complicated you want to get. Are you a hobbyist, professional? Some programs will not use above 4GB or 8GB of ram. Knowing your software requirements can save you some money (that being said… 16GB will never hurt).
I would focus more on the graphics card. Put your money here. It is most noticeable when doing something like rotating your model. Some of my more complex models have dropped me down to 1 frame a second (very slow rotate), but then, I don’t have the fastest video card. If you decide to use a program like Blender (which I use), stick to an NVidia card since that is the only one that’s fully supported. Again - know your software requirements and recommendations.
Once you decide what software you might want to try, look for requirements and recommendations from the producer. For example, Fusion 360 has a requirements page 6.
It can be confusion and please note that you never really want to go with a minimum requirement. It will be cheaper and it will work, but it will be frustrating because it will be slow.
SolidWorks! Very easy to use! Very Powerful!