I'm guessing you want to run a Bitcoin Core node, not program one from scratch, form my understanding of what you said.

Well basically, you'd need to open the RPC port and make it public (with no username/password). And use the command-line option to disable the wallet subsystem so people don't use it, and that's about it.

But you may want to put it behind a web server and use some rate-limiting inside so that it doesn't get overloaded.