What camera should I use to start? The best camera is...

The one you can afford and not miss the money. 'But what about specs? Brands? Resolution!?'

And all of that doesn't matter a damn. I have a Sony A7 currently and like all other cameras, if I am not filming, it isn't living up to its purpose. A 4K HD camera isn't worth shit if it isn't filming. Start with what you can comfortably afford and shoot with it. Try to pull every bit of pixel juice goodness out of it. Make something. If you make something with your heart in it and continue to do that, eventually you can worry about gear.

For now, Canon and Sony make great video DSLR's and you can get started for $400* and go up from there.

Blackmagic Design Production Camera 4K with EF Mount
$2,649.00 $2,750.00
Blackmagic Design

*$400 is for a last years model or generation, not the current. Normally current gen cameras are a lot more expensive. You are starting, don't dump everything you have into something that might now work out.