@hoergen on Friendica I'd like to search for posts with, for example, a #tag. Instead of searching only on the local network, I'd like to search for this tag across all the fediverse. For example the #newhere tag.
PeerTube has an opt-in system where the search bar of an instance can be used to search all videos on participating instances (the vidiverse), beyond those that are locally known. Then there's also https://sepiasearch.org/ for searching all PeerTube videos. I think people sometimes wish the search bars in other fediverse software work the same.
Welcome! Nice to have you with us! New here? Start with following some tags and people! Go check out public stream on your server to find some. Keep adding more people and tags, you can always unfollow something you don't want later.
Also do share your own stuff - write posts, upload images and add relevant tags so similar-minded people can easily find it. Consider filling out your profile as well, no personal data required. Any questions? Don't hesitate to ask, we are here to help.
As @Hypolite Petovan explained to you, your node can only search what it is aware of (means in local database only). Otherwise any search query needs to be replicated to all nodes causing a huge load on the network. There is an other way. You can subscribe to hashtags like e.g. #Friendica or #Fediverse or whatever you want. When your node has the relay enabled, it will include posts in your timeline with that hashtags you subscribed on.
You can "subscribe" to those tags by clicking on them (at least under Friendica) and click "follow" then. That's a healthier way (for the node network) instead of actively searching all nodes and causing a lot load on them. Don't see only yourself searching all alone, other users search, too!