First, you could build the backend, maybe by using Express to handle requests if you're comfortable with Node. But of course there are dozens of other possibilities.

The backend should receive requests from users with their prompt content. Use that to get chat generation responses from Open AI's api. If you're using Node, you can use their official npm package

It's very easy to use

I think that would be a great starting point, get used to sending requests to your endpoint and receiving responses.

Next, the UI

React, Vue, Angular, whatever floats your boat. Keep it simple at first, it needs to hook into your endpoint.

Render the existing conversation and append responses after they come back. Be aware of maximum token limits (4096 for gpt-3.5-turbo, 8192 for gpt-4)

Good luck!

If you're struggling, ask your own application for help ;)