What resources does Apple offer (such as listings or conferences) to help us promote our app?

There are no more Apple's developers conferences.

For the listings, AFAIK, that's the appstore.

There are a few developer conferences like this, but Apple does not sponsor any of them. Generally, Apple offers very little in terms of promotion. Certain apps may be "featured" in the App Store, but people on the internet say that you have to "know somebody" at Apple to get this service.

I think the World Wide Developers Conference (WWDC) is the only one Apple sponsors. It's expensive and I believe they use a lottery to decide who gets to purchase tickets.

However, you will probably find developers' clubs near to you (aka homebrew computer club of the time). That may be a very good start.