Searching in Pixelfed is very poorly implemented at the moment. When using the web interface, the search bar at the top of the screen is used to search for accounts to follow

If you click on a hashtag on an existing image, Pixelfed will show you all the posts on the server with the same hashtag.

However, if you want to type a hashtag to search for, you must manipulate the web address in your browser!

As an example, if you are using pixelfed.social as your Pixelfed server, change the address bar to:

https://pixelfed.social/i/web/hashtag/trees

where 'trees' is the hashtag you are looking for.

You can then use the Follow button to have posts with this tag appear in your own timeline in the future.