How to use HttpService for recieving Http requests?

HttpService can only send http requests, so without having to make my own website, how can I have HttpService recieve requests?

You can't

In order to have it receive requests, Roblox would have to somehow expose a request handle for that particular server, and you'd somehow have to tell something what aforementioned handle is. You're going to need a webserver, and you're going to need long-polling.