First of all, sorry for my bad English.
When I select the text, stars and hashes appear, and they also show up when I copy it. I don’t want these to appear. How can I fix this?
Those stars and hashes are “text format” for Markdown files. So, they need to be there 
But you can use … for example … Pandoc plugin, which allows you to export in multiple formats, including “plain text”, which looks like what you’re looking for.
And, regarding selecting text, you need to switch to Reading mode if you don’t want to see them while selecting text. But they will still be copied with the usual Ctrl + C
Cheers, Marko 