Looking to get started with API, and begin streamlining and optimizing my SW experience.

Where's the best place to begin?  Any really good tutorials out there?  Lots of videos on YouTube, however it seems that many of them just jump right in to the middle of something though and assume that the viewer already knows quite a bit.

For my first API project, I'd like to make a very quick way to change a drawing dimension unit(s) from fraction to decimal and be able to enter how many decimal places....then build from there.

Thanks for any help on this, as always.