I have an idea! I would like to create a phone holder that attaches to my forearm and allows me to turn the phone 90% but can lock into that position. Think of it as my phone as a wrist watch that is horizontal but if I want I could rotate it to look at it vertically.
I don't care if someone attempts designing something with this idea. My question is what software would I need to start building a design?
I design, build and teach using MatterControl, all-in-one CAD/slice/print.
if you are a complete noob, avoid tinkercad.. I started with designspark mechanical and never looked back... it is free, extremely powerful. and best of all intuitive... it can be nearly mastered in a matter of hours. it was designed for those who want to engineer stuff that don't have a CAD background...
For a functional model, you really don't want to use bender. That's great for organic models. For functional models like you are looking to do, you need a CAD program. There are many good ones and some have already been suggested. I'll throw my favorite in the hat - FreeCAD. It's free (duh, lol) and it's multi-platform. You can use it in linuc (like I do) or windows or (I think) mac, as well.
We I started designing my own stuff for 3D printing I start with CAD programs but I mostly use a 3d animation program called blender now. The reason is that most CAD are great for most non organic but for things like ship hall I find them vary limiting. For organic items they are useless.
TinkerCAD is good for beginners, since it uses basic shapes and operations, but for this project you might want something more powerful.
Fusion360 is commonly used since it's free for personal use or for startup companies below a certain revenue threshold, and is a very powerful software, even with with the free version. Personally I find the workflow very intuitive and quick to build things with, although I have been using it for a few years now. There are many tutorials online for Fusion as well. The caveat being that if you decide to start using it for commercial use it requires either paying for a licence or applying for the startup licence.
If you have intentions for using CAD commercially, you might be better off with FreeCAD, although it's less intuitive and takes a bit longer to model things in my experience, but there are no commercial licences required so no money or concerns about commercial use of your designs (hence FreeCAD).
There are other CAD programmes but those are the ones I have experience with (also OpenSCAD but that's quite different since it is script based and more suited to simpler parametric objects) so only the ones I can attest to.
The other answers here are good so.... to not answer your question, I saw something pretty similar to what you're describing in a contest currently running on Printables. Maybe you could remix it...
The first thing is to decide what TYPE of CAD program you would rather use.
I use OpenSCAD, because I am more comfortable with it than with a GUI.
OpenSCAD is like a programming language.
You can make it in any program for building 3d models.
For such projects the best may be CAD program (like FreeCAD or Fusion 360 [free for personal use])
Chose one and search for tutorials for it.