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.
Hi, I am surprised that nobody mentioned 123d design from autodesk, it is in my opinion the best entry level software for 3D printing, it is free, and once you master it it is a breeze to jump to Fusion 360 since the interfaces are very similar. Cheers!
I have used most of the popular cad softwares until a year ago when I purchased Moi3D and absolutely love it. I don’t even use Sketchup anymore except for architectural design o renderings. It is very easy to use, very accurate and let`s you can make organic shapes. It costs around $300 and for me it was worth every penny.
Hi Chris,
I am using CATIA V5. It’s one of the most powerful CAD tools you can find, but it’s not so easy to use.
It takes a little bit of time until you get to learn it, but once you do everything becomes possible :))
Hi Chris,
I would suggest tinkercad, very nice integration between 123Dapp from Autodesk!
Sketcup is very user friendly and with some pugins it works fine
www.shapr3d.com 2 runs only on the ipad Pro but it is a revelation in CAD. See youtube movies on their website. I learned it in about 2 hours time. Very similar as Tinkercad but you draw with a pencil on the ipad and its very natural. Further it has some very interesting features like chamfer round or square depending on the direction you pull the pencil. Unlike Tinkercad, their measures make sense, from the center of a point to the center of another point.
Sketcup is very user friendly and with some pugins it works fine
I suggest you use Onshape www.onshape.com 4 - it’s a FREE CAD package on the cloud from the team behind Solidworks.
It’s much more powerful than sketchup, ideal for engineering design.
SolidWorks! Very easy to use! Very Powerful!