I'd like to create a single online API endpoint that returns TRUE or FALSE. What tools would you use?

Basically, I'd like to have a flag, TRUE or FALSE, setup as an online endpoint. I'd like it to be online so I can have a set percentage of the time that it returns TRUE, and in real time change this percentage for all users of my software.

I've never done any server side work before. I expect around 100,000 calls to the endpoint every day.

Is there a simple tool that I could use to do the job? How would you go about doing this, where would you get started? I'm more than prepared to put a bit of time implementing this myself, I just don't know where to get started.