Ubuntu
 

I know some of you use Ubuntu for your computer operating system and I want to build a new computer, mine is old. Could someone recommend a good motherboard, it seems like all new motherboards will only load windows. I don't need a powerful computer as I don't play games etc.
I do use a 3D cad program. Thanks

Ubuntu is a word in many African dialects that means "idiot who can't install Debian" :D

Also, Mint is nicer - you may prefer it to bog standard Ubuntu....

That said, been a long time since I've built a computer. Mobo choice will dictate what type of CPU and RAM (and how much of it) you end up running...

I'd start with Tom's Hardware "best build for $x" series - Best PC Builds of 2017 - Build a Gaming PC on Any Budget

Really? That seems weird. I haven't researched, but I have to think that you may be mistaken or misunderstanding.

Ubuntu motherboards
 

I am a novice when it comes to computers but the new motherboards don't have a "bios", they use "uefi". These new boards are set to load only windows. I may be wrong about this but I know a new laptop my son had built would ONLY run windows.

Linux supports UEFI just fine.

Where you will run into issues is with DRM and TCP (trusted computing) which require a signed kernel. Even then, there will be specific Linux distros that provide that - probably RedHat

The consumer version of RedHat is CentOS, but I don't know if it includes a signed Kernel or not.