After I publish a post where does it go

You can just delete the posts from your wordpress site, but you are likely going to want to look at the media section of your wp site, as if you have many videos or photos on your site, those will take more space than just a post. When in your wp admin panel, on the left you should see a nav menu with one option being media. Delete what you don’t need out of there and see if it frees up the space you need. The posts will go into the wp database, so it’s better to delete them out from wp itself rather than editing the database.