Follow people.

And if you would like to boost it, join a relay. But check which relay, make sure it's content you want. And prepare for increased server load and media storage!

It's my understanding that the way mastodon works is that initially only toots on your instance can be seen by people on the instance. Of you want other content, a user would need to search for and follow users on other instances, then those other users' toots can show up on people's feeds in your instance. That's why bigger instances have more content for new users out of the box.

I could be wrong as I haven't made my own instance and I haven't seen the source code, but this is how it was explained to me.

I suggest adding some relays otherwise the only posts reaching your server will be things you yourself have followed, which makes hashtags a bit useless.

My current active relay list is: https://bigrelay.social/inbox https://relay.toot.io/inbox https://relay.intahnet.co.uk/inbox

But those are just what works for me.. not any particular recommendation.

Then follow lots of interesting people. Look on other servers if you're stuck, eg. the live feeds on mastodon.social. The more you follow the more conversations you'll see and find more interesting people. People who boost a lot are a goldmine for discovery (some people make a hobby of boosting eg. https://mastodon.social/@lisamelton).